Signs of independence among key U.S. senators are continuing to percolate out of Washington, as President Donald Trump’s presumably pending impeachment trial will again dominate the nation’s political headlines in coming weeks.

On Monday, Sen. Susan Collins, R-Maine, slammed fellow Sens. Elizabeth Warren and Mitch McConnell for making comments ahead of the impeachment trial before it’s even been officially scheduled.

"It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us because each of us will take an oath, an oath that I take very seriously, to render impartial justice," Collins told Maine Public Radio.

9. President Donald Trump declares a national emergency at the southern border. 8. The Robert Mueller report on possible Russian interference in the 2016 presidential election is released. 7. President Donald Trump releases the transcript of his phone call to the Ukrainian president. 6. The partial federal government shutdown reached a near-historic record. 5. Democrats launch public impeachment hearings aimed at President Donald Trump. 4. A federal court blocks Georgia's controversial new abortion bill f

Collins joined Sen. Lisa Murkowski, R-Alaska, in criticizing McConnell's comments earlier this month the Senate will work in "total coordination" with the White House in developing Trump's defense.

U.S. House Democrats have drafted two articles of impeachment against President Donald Trump. House leaders are charging the president with abuse of power and obstruction of Congress. They announced the articles of impeachment on Dec. 10, 2019. A full House impeachment vote could come before Christmas. If passed, the president would face a Senate trial in 2020, a presidential election year.

Collins, who said she is open to calling witnesses in the trial, said, “Democrats like Elizabeth Warren (are) saying that the president should be impeached, found guilty and removed from office. I’ve heard the Senate majority leader saying that he’s taking his cues from the White House. There are senators on both sides of the aisle, who, to me, are not giving the appearance of and the reality of judging that’s in an impartial way.”

In a recent interview with FOX News’ Sean Hannity, McConnell, a Kentucky Republican, described his planning with the White House.

“We’ll be working through this process, hopefully in a fairly short period of time in total coordination with the White House counsel’s office and the people who are representing the president as well as the Senate,” McConnell said.

The Impeachment Process: Explained.Impeachment is the political process of removinga civil officer from office for reasons of “treason,bribery, or other high crimes and misdemeanors.”.It can be brought against eventhe highest officials, such as thePresident and Vice President. .The process begins in the U.S. House of Representatives, where a suggestion to impeach must be made and then authorized to proceed by the Speaker of the House.From there, the speaker chooses to move the inquiry to either a

Collins is one of four GOP senators being pressured by Republican anti-Trumpers to allow witnesses to testify during the president's trial.

»MORE: Anti-Trump groups ratcheting up pressure on GOP senators

On Monday, Senate Minority Leader Chuck Schumer, D-N.Y., renewed his call for witnesses and documents after a Sunday New York Times report detailed how Trump aides handled the president's alleged request to withhold Ukrainian military aid.

Trump is only the third sitting president in American history to be impeached, joining Andrew Johnson and Bill Clinton.

Impeachment was established in the U.S. Constitution as a way to accuse a president of a crime and then hold a trial to determine if guilty. The first step requires a U.S. House member to introduce an impeachment resolution. The House speaker directs the judiciary committee to hold a hearing to decide whether to put the full measure to a vote by the full chamber. A majority of the committee must approve the resolution. If approved, it moves to a full vote on the House floor. If a majority of the House vot

The two articles of impeachment by House Democrats — abuse of power and obstruction of Congress — point to Trump pressuring Ukraine to investigate 2020 political rival Joe Biden while withholding as leverage military aid the country relies to counter Russia as well as his efforts to block the House investigation.

»MORE: Who are the major players in a Trump impeachment trial?

With Congress now on its holiday break, House Speaker Nancy Pelosi still hasn't sent Trump's articles of impeachment over to the Senate, the next step in an impeachment trial.

Despite Democrats' professed sense of urgency in passing House impeachment articles against the president, Pelosi has delayed sending the charges over to the Senate. Pelosi is demanding information from the Senate on how it plans to conduct Trump's trial and hopes to give Schumer more leverage in talks with McConnell.

The Constitution requires a two-thirds majority in the Senate to convict in an impeachment trial, thus making Trump’s actual removal from office highly unlikely in the GOP-controlled Senate.