U.S. Sen. Johnny Isakson's decision to retire at the end of the year paves the way for two separate U.S. Senate elections in 2020 – and two separate battles over who should replace the third-term Republican.

The first will involve quiet jockeying to sway Gov. Brian Kemp, a Republican who will soon tap someone to fill out the next year of Isakson’s term. Already, Republicans are making behind-the-scenes moves to position themselves for an appointment – or rule themselves out.

The second will be a much noisier Democratic race to fill the seat. Three Democrats have already launched challenges to U.S. Sen. David Perdue, who is seeking a second term next year, and the list of potential candidates for Isakson's seat is growing.

It’s likely to be Kemp’s most important political decision of the year – maybe of his first term. It’s also one of his most complex. Not only will his pick share the ballot with President Donald Trump and Perdue in 2020, he or she would also be on the same ticket with Kemp in 2022 if Republicans hold the seat.

The outcome has vast implications. Democrats already consider Georgia a must-win to flip control of the Senate, where Republicans have a 53-47 edge. A second Senate race will only mean more attention and money focused on Georgia.

» MoreIsakson's retirement makes Georgia 'ground zero' for politics in 2020

» Photos: Johnny Isakson through the years

» Timeline: Key moments in Johnny Isakson's public life

» Related: How will U.S. Sen. Johnny Isakson's replacement be chosen?

“Georgia is now ground zero in national politics and there’s no doubt about it,” said Chip Lake, a veteran Republican strategist and adviser to Lt. Gov. Geoff Duncan. “How often can we say we’re the center of the political universe? Well, right now we are.”

The governor was succinct when asked Wednesday by The Atlanta Journal-Constitution what qualities he's seeking in a candidate for Isakson's successor: "Somebody who is going to represent our values in the state, that's going to fight for us, fight for Georgians in the U.S. Senate."

Kemp's advisers say they are starting from square one and that they won't be beholden to anyone from his 2018 campaign. That's important because he picked up several key endorsements, most notably Trump's support at the behest of Agriculture Secretary Sonny Perdue.

He’ll be among the candidates rumored for the job, as will other high-profile Republican officials: Attorney General Chris Carr, Lt. Gov. Geoff Duncan and U.S. Reps. Doug Collins and Drew Ferguson.

But Kemp seems just as likely to select a Republican who doesn’t fit the traditional Georgia GOP mold.

The governor has already surprised critics with his early appointments, including his pick of acting Insurance Commissioner John King, a low-profile local police chief who became the state's first Hispanic constitutional officer. He's also made a string of diverse, history-making selections for judicial posts.

State Republicans were clobbered in the Atlanta suburbs the last two elections, suffering a defection of suburban white women. He might aim for an outsider candidate – or at least a lesser-known figure – who can appeal to suburbanites, women or minorities.

Gov. Brian Kemp. ALYSSA POINTER/ALYSSA.POINTER@AJC.COM

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Crowded contest

He’ll have to weigh a range of other fraught considerations. Because a special election will determine who serves the final two years of Isakson’s term, there won’t be a party primary to hash out a nominee. Instead, candidates from all political parties will share the same November ballot. That also means far higher turnout – and far more unpredictability.

Whoever Kemp picks will have to scare off – or fend off – rival Republicans in the free-for-all contest. He’ll also seek someone who can raise campaign cash and unite the GOP base against Democrats who see Georgia as a swing state.

The format of the race could be an opening for Democrats squeamish about challenging Perdue, a formidable fundraiser and Trump ally, but more willing to take their chances against a GOP appointee.

More moderate Democrats who wouldn’t stand a chance in a party primary also might figure they have a better shot with a broader November electorate.

It’s likely to be a jumbled race no matter what. The last federal special election in Georgia was the 2017 contest for Georgia’s 6th Congressional District, when 18 candidates competed to replace U.S. Rep. Tom Price. That contest, won by Republican Karen Handel, was the most expensive U.S. House race in history.

And only a few top potential candidates have ruled out a race, among them Stacey Abrams, who seems likely to position herself for a 2022 rematch against Kemp; and Nick Ayers, a former top aide to Vice President Mike Pence who will stay in the private sector.

Meanwhile, the list of potential candidates who are credible contenders for the seat is only growing. And the maneuvering is well underway.

Here’s a look at some of the candidates who could emerge.

Republicans:

October 12, 2016, Atlanta - Chris Carr responds to a reporters question during a press conference where he was appointed the next Attorney General following the Board of Regents vote to hire current Attorney General Sam Olens as president of Kennesaw State University in Atlanta, on Wednesday, October 12, 2016. Carr will take his new position on November 1.(DAVID BARNES / DAVID.BARNES@AJC.COM)

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Georgia Attorney General Chris Carr

Carr might as well be a member of Isakson’s family. He was Isakson’s top aide before he became Georgia’s economic development commissioner, and he’s married to the senator’s current chief of staff. He’s fresh off a close election victory to win a full term in 2018. He’s said to be not actively campaigning for the role.

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U.S. Rep. Doug Collins

The charismatic congressman from Gainesville has emerged as a hero to Trump supporters because of his sharp defense of the president in Congressional hearings. The ranking Republican on the House Judiciary Committee, Collins represents the most conservative district in Georgia and is a robust fundraiser with close ties to the White House and the Perdue family. Collins has applied for the role.

Lt. Gov. Geoff Duncan created a task force to study health care access and costs. It could lead to legislation being proposed in the upcoming session of the General Assembly. (ALYSSA POINTER/ALYSSA.POINTER@AJC.COM)

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Lt. Gov. Geoff Duncan

The former Georgia Tech baseball star waged a longshot bid for lieutenant governor last year – and scored an upset victory over the Republican favorite before a narrow general election victory. Since then, he's emerged as a staunch Kemp ally, and their partnership shaped the legislative session.

U.S. Rep. Drew Ferguson. AJC file.

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U.S. Rep. Drew Ferguson

A rising star in Georgia’s congressional ranks, Ferguson won a senior spot in House GOP leadership last year and is seen as one of the state’s most ambitious and energetic politicians.

U.S. Rep. Tom Graves, R-Ranger, listens to now-active White House Chief of Staff Mick Mulvaney give testimony during an appropriations hearing on the budget for the Office of Management and Budget on June 21, 2017 (Photo by Astrid Riecken/Getty Images)

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U.S. Rep. Tom Graves

The former state lawmaker is the senior-most Republican in the Georgia delegation, and he's transformed his reputation from a rabble-rousing bomb thrower to a more pragmatic politician who has become one of the state's more powerful politicians.

U.S. Rep. Karen Handel, R-Roswell, on election night last year. Curtis Compton,ccompton@ajc.com

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Former U.S. Rep. Karen Handel

Handel might have one of the best-known Republican names in Georgia. She served as secretary of state and then ran unsuccessfully for governor in 2010 and U.S. Senate in 2014. Her political comeback came in 2017 when she won the nationally-watched 6th District special election, a seat she lost last year to Democrat Lucy McBath. She’s angling for a rematch, but people close to her say she wouldn’t rule out a Senate bid.

State House Speaker Pro Tem Jan Jones, R-Milton, said she didn’t think a bill to eliminate Georgia’s 4 percent sales tax on menstrual items addresses the goal of removing the burden of taxing women who struggle to afford basic necessities. She moved for funds to be set aside for schools in low-income areas to distribute the products to girls. She also wants county health departments to get more money to help low-income women. Bob Andres / bandres@ajc.com

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Georgia House Speaker Pro Tem Jan Jones

One of the most powerful Republicans in the Georgia House, the Milton lawmaker is an influential, if sometimes under-the-radar, force in every major legislative decision under the Gold Dome. First elected in 2002, Jones has cultivated a broad base of support across the Atlanta suburbs. She has applied for the role.

072214 ATLANTA: U.S. Senate candidate Jack Kingston hugs his wife Libby and waves to supporters at the end of his address conceding defeat to David Perdue at his election night party at the Georgia Tech Hotel & Conference Center on Tuesday, July 22, 2014, in Atlanta.   CURTIS COMPTON / CCOMPTON@AJC.COM

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Former U.S. Rep. Jack Kingston

The ex-Savannah congressman was the runner-up to Perdue in the 2014 GOP runoff, and has transformed himself into a cable TV pundit - and defender of Trump - since his defeat. He'd focus a campaign on his grassroots organization skills and ties to Trump; he'd get dinged for his recent lobbyist work

Atlanta Dream co-owner Kelly Loeffler cheers for the team during 2nd half action in the home opener at Philips Arena in Atlanta on Friday, May 25, 2012. Atlanta Dream won 100 - 74 over the New York Liberty. HYOSUB SHIN / HSHIN@AJC.COM

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Business executive Kelly Loeffler

After passing on a U.S. Senate run in 2014, Loeffler has stayed in the public spotlight as part-owner of the WNBA's Atlanta Dream and chief executive of Bakkt, a digital firm that's a subsidiary to the Intercontinental Exchange.

2/4/19 - Atlanta - U.S. Attorney Byung J. “BJay” Pak speaks to the Rotary Club of Atlanta on Monday, February 4. This is Pak’s first address to the civic group and his most public forum since he took office more than a year ago. Pak discussed the history of the attorney’s office and top priorities within his office. EMILY HANEY / emily.haney@ajc.com

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U.S. Attorney BJay Pak

An ex-Gwinnett lawmaker, Pak was tapped in 2017 to head the U.S. Attorney's office for the busy district that spans metro Atlanta and North Georgia. He's grabbed headlines for a flurry of high-profile cases, including the ongoing corruption probe into Atlanta City Hall. He was Georgia's first Korean-American legislator and is seen as a likely GOP candidate for higher office in the future. He has said he's not interested in the job.

04/05/2019 -- Newnan, Georgia -- United States Secretary of Agriculture Sonny Perdue (center) arrives at the U.S. Department of Agriculture's National Detector Dog Training Facility in Newnan, Friday, April 5, 2019.  (ALYSSA POINTER/ALYSSA.POINTER@AJC.COM)

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Agriculture Secretary Sonny Perdue

The former Georgia governor was one of the Kemp’s most significant allies in last year’s election, and is credited with convincing Trump to wade into the race. It’s (highly) unlikely that he’d want to jump back into elected office, but he remains popular with the GOP base and would be formidable if he does.

Democrats

DeKalb County District Attorney Sherry Boston has raised problems with Georgia’s new anti-abortion “heartbeat” law, House Bill 481. “As district attorney with charging discretion, I will not prosecute individuals pursuant to HB 481 given its ambiguity and constitutional concerns,” Boston said. (DAVID BARNES / SPECIAL)

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DeKalb County District Attorney Sherry Boston

The top prosecutor in Georgia's most Democratic stronghold has scored a string of headlines for her criminal justice stances. She's said she won't prosecute women under Georgia's controversial new anti-abortion law and advocated for other changes to the state's criminal code. She was said to be conferring with national Democrats on Wednesday about the race.

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Former gubernatorial nominee Jason Carter

A former state senator, Carter was the runner-up in the 2014 race against Gov. Nathan Deal with a campaign that focused on preserving the HOPE scholarship and appealing to more moderate voters. Carter, a grandson of former President Jimmy Carter, has stayed politically active since his defeat, and could benefit from the free-for-all format of the race.

Georgia Sen. Jen Jordan, D-Atlanta, testifies before the Senate Judiciary Committee on April 9, 2019. Photo courtesy of the U.S. Senate Photographic Studio.

Credit: U.S. Senate Photographic Studio,Dan Rios

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State Sen. Jen Jordan

The Sandy Springs Democrat has emerged as a force in Georgia politics since flipping a Senate seat in a 2017  special election. She's one of the sharpest critics of Georgia's new anti-abortion laws, and has led the charge to close down a plant in her district that's leaking cancer-causing gasses.

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Matt Lieberman

Lieberman, an Atlanta educator and entrepreneur, is said to be "seriously considering" a run for Isakson's post. He's the son of former U.S. Sen. Joseph Lieberman and could wage an outsider campaign for the seat, though he's virtually unknown in Democratic circles.

Sixth District Congresswoman Lucy McBath amended her financial disclosure form.

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U.S. Rep. Lucy McBath

The first-term Marietta congressman was the biggest Democratic victor in the 2018 midterm, ousting Republican Karen Handel in the race for the north Atlanta district. She became a nationally-known gun control advocate after her son's shooting death, and she would likely have the support of billionaire Michael Bloomberg if she ran. She has said she will not run.

FILE - This May 20, 2014, file photo shows Democratic candidate for U.S. Senate, Michelle Nunn in Atlanta as she speaks to her supporters after her primary win was announced at an election-night watch party. Minutes after David Perdue won his Republican Senate primary in Georgia, a grainy image of Nunn was on Atlanta TV as a faceless voice slammed her as a rubber stamp for President Barack Obama. In turn, Democrats portray Perdue as a greedy executive who laid off workers. Itís all a clear indicator of what the next three months hold for a first-time Democratic candidate who may be the party's only hope for picking off a GOP-held Senate seat in November. (AP Photos/Akili-Casundria Ramsess, File)

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Nonprofit executive Michelle Nunn

The head of Atlanta-based CARE and a 2014 Democratic nominee for U.S. Senate, Nunn waged a centrist-friendly campaign against Perdue five years ago. Nunn, daughter of former U.S. Sen. Sam Nunn, has not ruled out another bid. A strategist close to Nunn said she's seriously considering a bid.

June 6, 2019 Atlanta - Dekalb  County CEO Michael Thurmond speaks during  the DNC's IWillVote Gala at Flourish Atlanta on Thursday, June 6, 2019. White House hopefuls court Georgia: Presidential candidates swarmed Atlanta on Thursday for a convention geared at African-American strategists and a joint fundraiser featuring four of the best known Democratic contenders on the same stage. HYOSUB SHIN / HSHIN@AJC.COM

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DeKalb County Chief Executive Michael Thurmond

Thurmond is one of the most experienced politicians in Georgia. The former state legislator was the first African American elected to a statewide office in Georgia without prior appointment, serving three stints as labor commissioner. After losing a U.S. Senate bid to Isakson in 2010, he served as DeKalb's superintendent and easily won election in 2016 to be the county's CEO. He hasn't ruled out a run.

The Rev. Raphael Warnock, pastor of Ebenezer Baptist Church, sees the issue of mass incarceration as the struggle for America’s soul. HYOSUB SHIN / HYOSUB.SHIN@AJC.COM

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The Rev. Raphael Warnock

The senior pastor of the Rev. Martin Luther King Jr.'s historic congregation, Ebenezer Baptist Church, flirted with a bid for U.S. Senate in 2016 before opting against it. Party leaders see him as a top contender in 2020 who could focus his bid on voting rights and income equality, though it's not clear if he's interested.

State Sen. Nikema Williams, chairwoman of the Democratic Party of Georgia, speaks during a press conference at the state Capitol in Atlanta on Nov. 14, 2018. (Alyssa Pointer / alyssa.pointer@ajc.com)

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Democratic Party of Georgia Chairwoman Nikema Williams

After years as a party activist and organizer, Williams won an Atlanta-based state Senate seat in 2017 and, less than two years later, was elected to lead the state party. She's advocated for a more aggressive approach in state politics, including urging presidential candidates to pour resources into Georgia and actively recruiting municipal candidates who run in nonpartisan races.  Williams also made national headlines when she was arrested during a special legislative session for participating in a Capitol protest; those charges were later dropped.

(AP)

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Former acting U.S. Attorney general Sally Quillian Yates

Yates became a household name overnight in January 2017 when, as the caretaker attorney general, she refused to enforce Trump’s travel ban. Trump swiftly fired her. She’s continued to raise alarms about Trump’s policies since returning to private practice in Atlanta. She dismissed talk of running against Perdue, but national Democrats are boosting efforts to recruit her for Isakson’s spot. It might be a “pipe dream,” as one strategist put it, but it’s generating lots of buzz.