Senate Republicans are considering punting a decision about what, if any, witnesses to allow during an impeachment trial until after the proceeding starts.

The discussions would buy more time for negotiations, both among senators and with the White House, about what is emerging as one of the most contentious points of the looming Senate proceeding.

"My belief is that what happened last time probably produces the logical way to decide whether you need witnesses or not, which is present your case ... and then the Senate can decide or not, just like they did last time, whether we need witnesses or not," said Sen. Roy Blunt (R-Mo.), a member of the GOP leadership.

Blunt added that he expected that process was "likely" to be replicated during a Trump trial.

Meanwhile, asked if he expected a final decision on witnesses after the start of the trial, Sen. John Thune (S.D.), the No. 2 Senate Republican, said he expected a decision on whether or not to call witnesses to be made "down the road."

"It will be whatever, after having heard initial arguments, 51 senators decide they want to do," Thune said.

The talk of trying to separate a decision on witnesses from the broader negotiations on the process for an impeachment trial comes as the House is poised to vote on articles of impeachment this week, setting up a trial that will start in January.

Senate Majority Leader Mitch McConnell (R-Ky.) and Senate Minority Leader Charles Schumer (D-N.Y.) have not yet started negotiating on the parameters of what the Senate proceeding will look like.

But Democrats are stepping up their efforts to put the squeeze on Republicans, with Schumer publicly releasing a letter to McConnell on Sunday, and sending it to the other 98 Senate offices on Monday, outlining the Democrats' opening position on the looming talks.

Schumer, in his letter, said that Democrats want to call acting White House chief of staff Mick Mulvaney, his senior adviser Robert Blair, former national security adviser John Bolton, and Office of Management and Budget staffer Michael Duffey to testify as part of a Senate trial.

Schumer declined to negotiate in public on Monday, sidestepping questions about whether Democrats would agree to taped depositions or what witnesses called by Republicans they would allow. Instead he tried to put the onus on Republicans to agree to a "fair" trial once the articles of impeachment come over to the Senate.

"Trials have witnesses.That's what trials are all about," Schumer told reporters. "To engage a trial without the facts coming out is to engage in a cover-up."

He added that if "McConnell doesn't hold a full and fair trial, the American people will rightly ask what are you, Leader McConnell, and what is President Trump hiding?"

In a sticking point, Schumer wants one resolution that would cover both procedure and calling specific witnesses that would be passed at the start of the trial under his plan.

Republicans have been noncommittal to Schumer's letter. McConnell sidestepped commenting on Monday. A spokesman for McConnell said that the majority leader has said that he plans to sit down with Schumer "soon" to discuss the trial process.

"That timeline has not changed," the McConnell spokesman added.

McConnell has hinted that the Senate will not make a decision about what witnesses will testify - either live or behind closed doors - until after both the president's team and House impeachment managers are able to make their initial arguments in the trial.

"It could go down the path of calling witnesses and basically having another trial, or it could decide, and again 51 members could make that decision, that they have heard enough and believe they know what would happen and could move to vote on the two articles of impeachment sent over to us by the House. Those are the options. No decisions have been made yet," McConnell told reporters during a recent press conference.

Pressed for details on how witnesses would be handled, he added: "We will make that decision after we have heard the opening arguments."

During the impeachment trial of former President Clinton, senators voted 100-0 on a resolution laying out the process for a trial, but a vote on a subsequent resolution calling for specific witnesses broke down along party lines. The initial resolution outlining procedure passed on Jan. 8, 1999, while the second resolution calling specific witnesses passed on Jan. 28, 1999.

"Schumer's plan puts a couple things together as if they happened all at once last time, and they did not," Blunt added.

GOP aides also hammered Schumer on Monday. They noted that he was now calling for live witness testimony but voted against a resolution under the Clinton trial that allowed for subpoenas for key figures such as Monica Lewinsky, Sidney Blumenthal and Vernon Jordan Jr. to testify in 1999.

"The witnesses in '99 had already given grand jury testimony. We knew what they were to say. The four witnesses we have called have not been heard from. That is the difference, and it is a difference that is totally overwhelming," Schumer said, asked about his comments in 1999 compared to now.

Republicans are eyeing a shorter impeachment trial that would include no witnesses for either Trump's team or House impeachment managers, though they've stressed that no final decision has been made.

McConnell, who has not locked down a public position, voted to allow witnesses to take part in taped depositions during the Clinton trial.

It's put some Republicans at odds with some in the White House, who have reiterated that Trump wants House Intelligence Committee Chairman Adam Schiff (D-Calif.), Hunter Biden and the whistleblower at the center of the House impeachment inquiry to testify.

Absent some sort of larger deal, 51 senators will effectively be able to decide on the Senate floor who will testify and who will not, opening up the possibility that they could either lose votes on controversial nominees or that Democrats could try to peel off GOP senators.

With Republicans holding a 53-47 Senate majority, McConnell would need to hold together almost all of his caucus, including several senators facing tough reelection bids. Unlike in a normal Senate vote, Vice President Pence could not break a 50-50 tie. Senators expect a tie during the trial would result in a motion failing.

Sen. John Cornyn (R-Texas), an adviser to McConnell, warned there could be "unintended consequences" if senators agree to have live witnesses as part of an impeachment trial.

"Witnesses say the darndest things," Cornyn said.

But he added that he would support closed-door depositions for witnesses, though he did not point to specific individuals he thought needed to be deposed.

"Anybody that the impeachment managers want to present evidence from," he said. "I'm for them taking depositions from anybody who has knowledge of relevant facts."