Senate Republicans are barreling toward a high-profile fight on impeachment witnesses — one of the biggest wild cards of President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE’s trial.

With the House expected to send the articles across the Capitol as early as Wednesday, GOP senators are slated to pass a resolution on trial rules next Tuesday that would punt a decision on whether to call witnesses for roughly two weeks.

There are already signs of division in the Republican caucus over whether witnesses are needed and, if so, who would be on that list.

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Asked if he had concerns the witness debate could get “messy,” Sen. John Thune John Randolph ThuneThe Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump contradicts CDC director on vaccine, masks Senate GOP eyes early exit MORE (S.D.), the No. 2 Senate Republican, said “that’s the state of play.”

“If that door is opened … then both sides are obviously going to want to call witnesses, and then you’re going to have probably individual votes on individual potential witnesses that people want to call,” Thune said.

Democrats would need to peel off four GOP senators in order to call preferred witnesses such as former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE and acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE.

Thune stopped short of saying there are four Republicans who would vote to subpoena witnesses, saying he didn’t want to be “handicapped.” But he added Senate Republicans “have members … [that] want to hear from witnesses.”

The debate, which comes as lawmakers are finalizing the rules resolution, sets the stage for the kind of divisive floor fight GOP leaders have tried to avoid.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled McConnell accuses Democrats of sowing division by 'downplaying progress' on election security Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (R-Ky.) knocked Democrats’ request for witnesses beyond those who spoke with the House during its inquiry, arguing they are trying to turn the impeachment trial into a fishing expedition.

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“If the existing case is strong, there’s no need for the judge and the jury to reopen the investigation. If the existing case is weak, House Democrats should not have impeached in the first place,” McConnell said from the Senate floor.

He also sent a warning shot to Democrats, and his own caucus, when asked Tuesday during a weekly press conference about calling former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE’s son Hunter Biden to testify, saying: “I can’t imagine that only the witnesses that our Democratic colleagues would want to call would be called.”

The comments come after McConnell sent a similar warning last month, during an interview with Fox News Radio, that if Democrats successfully called a witness then he expected Republicans would want to hear from Hunter Biden and the whistleblower.

“You can see here that this would be a kind of mutual assured destruction episode that would go on for a long time,” McConnell said, adding that his preference would be for no witnesses.

On one side, Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) and conservative Republicans are warning they will force votes on controversial witnesses like Hunter Biden and the whistleblower if some of their colleagues support Democrats’ request for individuals like Bolton.

“I don’t think we should selectively call just witnesses that don’t like the president,” Paul said Tuesday, while specifying that his first preference is for no witnesses.

Paul added that if his GOP colleagues backed a subpoena for Bolton or other Democratic-supported witnesses and “vote against the president bringing in witnesses, it’s not going to really be very helpful for them with the Republican base.”

Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) added that if senators support calling a witness sought by House managers they also need to support calling Trump’s preferred witnesses.

“If the Senate decides to allow the prosecution to call yet more witnesses, after all the witnesses they had in the House, they now want to call more witnesses … the defense needs to be able to call at a minimum an equal number of witnesses, and witnesses of their choice,” Cruz told conservative radio host Hugh Hewitt on Tuesday.

The internal Republican haggling comes after McConnell announced that he has the votes to sideline Democrats and pass the rules for the trial with only Republicans.

But Democrats believe a steady stream of new reports, and Bolton’s public offer to testify, are helping build pressure on Republicans to support some witnesses — either live or in closed-door depositions. Bolton’s lawyer, in particular, has said he will have information relevant to the trial. Democrats are expected to force votes on witnesses at the beginning and middle of the trial.

“The American people want a fair trial in the Senate. The American people know that a trial without witnesses and documents is not a real trial, it’s a sham trial, and the American people will be able to tell the difference,” Senate Minority Leader Charles Schumer Chuck SchumerMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security Warren, Schumer introduce plan for next president to cancel ,000 in student debt Schumer lashes out at Trump over 'blue states' remark: 'What a disgrace' MORE (D-N.Y.) said.

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A small group of Republicans, including Sen. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally Gideon leads Collins by 12 points in Maine Senate race: poll Senate leaders quash talk of rank-and-file COVID-19 deal MORE (Maine), is working to ensure that language that leaves the door open to either side trying to call witnesses makes its way into the Senate rules resolution.

Collins — when asked about the argument that if one side successfully calls witnesses then the other should be able to as well — reiterated that she wants both sides to be able to request witnesses. A majority of the Senate, where Republicans hold a 53-47 advantage, has to vote to call a witness.

“Both sides should certainly have the right to call witnesses,” she said. “If you’re going to be fair, you … can’t just give that to one side.”

Sen. Mitt Romney Willard (Mitt) Mitt RomneyGOP-led panel to hear from former official who said Burisma was not a factor in US policy Joe Biden's dangerous view of 'normalcy' The electoral reality that the media ignores MORE (R-Utah) indicated this week that he would vote to hear from Bolton during the trial.

“I support the Clinton model, which means that we will have opening arguments first. Then we’ll have a vote on witnesses and at that stage I presume I’ll be voting in favor of hearing from John Bolton, perhaps among others. That could change,” Romney said.

Asked if he thought that obligated him to also support calling Hunter Biden or the whistleblower, he said he would make a decision if it comes to that.

“I’d presume if we’re going to have any witnesses, we’ll have witnesses from both sides,” he said.