Two key GOP senators are backing Senate Majority Leader Mitch McConnell’s insistence that President Donald Trump’s impeachment trial should resemble that of Bill Clinton’s in 1999.

On Monday, U.S. Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) agreed with McConnell that Trump's trial should follow Clinton's precedent and defer the process of calling additional witnesses, according to The Hill.

Senate Minority Leader Chuck Schumer (D-N.Y.) and other House Democrats have been demanding additional witnesses be called during a Trump trial. Senate scheduling of the trial has been stalled on Capitol Hill for weeks.

Donald Trump has become the third American president to be impeached. Trump has been charged with abuse of power and obstruction of Congress. Led by Democrats, the U.S. House of Representatives passed the articles of impeachment on Dec. 18, 2019. Trump will face trial in the GOP-controlled Senate in 2020, a presidential election year. Presidents Andrew Johnson and Bill Clinton were also impeached. Neither was removed from office.

Collins told reporters Monday evening the Senate should consider the question of subpoenaing additional witnesses only after House impeachment managers and Trump’s defense team present their opening arguments.

Earlier Monday, former White House national security adviser John Bolton said he would be willing to testify in Trump’s trial if subpoenaed by the Senate.

Trump impeachment trial: John Bolton says he will testify if subpoenaed

“We need to do what they did the last time they did this unfortunate process and that was to go through a first phase and then they reassessed after that,” Murkowski said.

The Senate is still waiting on House Speaker Nancy Pelosi to send over Trump's articles of impeachment, the next step in an impeachment trial.

Despite Democrats' professed sense of urgency in passing House impeachment articles against the president last month, Pelosi has delayed sending the charges over to the Senate and refused to name the House managers who would handle the trial until Senate GOP leaders meet her demands.

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. But neither Schumer nor McConnell has shown any sign of compromising on his respective positions.

»MORE: McConnell, Schumer still at odds over Trump impeachment trial

Republicans control 53 Senate seats and Democrats have 45, plus the support of two independents. Schumer needs at least four Republicans on his side to pass a resolution for the trial meeting his demands.

Schumer wants Bolton and acting White House chief of staff Mick Mulvaney; Robert Blair, a Mulvaney adviser; and Michael Duffey, an Office of Management and Budget associate director, to testify at Trump’s trial.

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 and Murkowski have been critical of McConnell in recent weeks. McConnell said he would coordinate Trump's defense in lockstep with the White House, prompting rebuttals from both senators.

»RELATED: More senators breaking ranks from their parties on impeachment

Collins is one of 14 current senators who were in office during Clinton's impeachment. There are also 13 sitting senators who voted on Clinton's late 1998 impeachment while serving in the House.

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

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.