WASHINGTON – Senate Democrats are asking that several key witnesses who declined to cooperate with House impeachment investigators be called as part of the Senate trial examining whether President Donald Trump should be removed from office

Senate Minority Leader Chuck Schumer, D-N.Y., outlined his priorities and a possible structure for a weekslong Senate trial in a three-page letter to Senate Majority Leader Mitch McConnell, R-Ky. It marked the first formal move between the two leaders to negotiate a compromise on an impeachment trial, a key step that will help establish how long a trial will run and whether witnesses would be called to testify as part of the process. 

If Schumer and McConnell do not come to a bipartisan compromise, a majority in the Senate could agree on a measure outlining a process or senators to simply introduce motions and vote on them as they go. 

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"Senate Democrats believe strongly, and I trust Senate Republicans agree, that this trial must be one that is fair, that considers all of the relevant facts, and that exercises the Senate’s 'sole Power of Impeachment' under the Constitution with integrity and dignity," Schumer writes in the letter, noting that it must be a process that passes the "fairness test with the American people."

"That is the great challenge for the Senate in the coming weeks," Schumer added. 

Doug Andres, a spokesman for McConnell, said that McConnell plans to meet with Schumer to discuss a deal.

"Leader McConnell has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon," Andres said. "That timeline has not changed."

Since Republicans hold a slim majority in the Senate (53 to 47), conservatives could hold more control over how the trial is run than Democrats. But, the thin margin paired with a number of Republicans facing tough races in the upcoming 2020 race, McConnell will be forced to walk a careful line and prevent the proceedings from becoming the spectacle some House Republicans have called for. 

In the letter, Schumer asks that Chief Justice John Roberts, the Supreme Court's chief judge who will be tasked with presiding over the trial, issue subpoenas for four key officials who were called by House Democrats and refused to appear. Those include Mick Mulvaney, acting White House chief of staff; John Bolton, former national security advisor; Robert Blair, senior advisor to the acting White House chief of staff; and Michael Duffey, associate director for national security, Office of Management and Budget. 

All four were named in closed-door testimony from more than a dozen witnesses. They are believed to have information key to the House investigation into whether Trump withheld about $400 million in military aid and a key White House meeting from Ukraine in exchange for the country working on two investigations that were helpful to him politically, charges that led to two articles of impeachment that the House will vote on later this week. 

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Bolton, in particular, has drawn the interest widespread interest due to his departure from the Trump administration, witnesses who testified he was troubled by the Ukraine pressure scheme and comments from his attorneys that hinted Bolton had information that could be key in the probe. 

Ultimately, Bolton failed to appear for his scheduled deposition. 

Along with the witnesses, Schumer asks that key documents be subpoenaed from the White House, Statement Department and Office of Management and Budget that could "shed additional light on the Administration’s decision-making regarding the delay in security assistance funding to Ukraine and its requests for certain investigations to be announced by the government of Ukraine."

The letter also marks the first sense of when a Senate trial could happen. Schumer asks that the process begin the week of Jan. 6 and cites the procedures used during the 1999 trial of President Bill Clinton, asking that House prosecutors and the president's counsel be given up to 24 hours to make their opening presentations and give senators a total of 16 hours to question House managers and the president's counsel. 

Schumer also asks that the House prosecutors and the president's counsel be given up to three hours each for final arguments. Finally, he asks that each senator be given a period of not more than 24 hours to deliberate before casting their verdict. 

The Senate agreed unanimously on procedures outlining how the 1999 impeachment trial into Clinton was held but absent from the resolution was the calling of specific witnesses. Schumer asks that everything in his letter be wrapped into one resolution, outlining exactly how the trial would run. 

Senate Republicans have expressed caution in calling witnesses in the trial, even though Trump had been adamant that several people be called, including the whistleblower and Joe Biden. The worry is calling witnesses would lengthen the process and open the door to more testimony, creating a televised partisan display. 

McConnell has not publicly announced his priorities for a Senate trial but on Thursday, he told Fox News that he was working in "total coordination with the White House counsel" on this, causing Democrats to attack McConnell and questions over whether the process would be fair. 

"Everything I do during this, I’m coordinating with White House Counsel," he said. "There will be no difference between the President’s position and our position as to how to handle this."

McConnell's comments came hours after he, White House counsel Pat Cipollone, and Legislative Affairs Director Eric Ueland met privately in the Majority Leader's office in the U.S. Capitol to discuss the matter. 

“The president deserves to have his case heard,” Ueland told reporters as he left McConnell’s office earlier Thursday. “We're having good, close communication and conversation with Senate Republicans." 

This article originally appeared on USA TODAY: Trump impeachment: Democrats want Bolton, Mulvaney in Senate trial