Republicans are threatening to weaponize a fight on Senate impeachment witnesses amid growing concerns that moderates within their caucus could help Democrats call 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 to testify.

After weeks of pledging that they would hold a quick trial with no witnesses from either side, Republicans — from Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) on down — are sending public warning shots that if their GOP colleagues open the door to Democratic witnesses they’ll respond in kind, forcing votes on a slew of controversial individuals.

The pressure tactics are the latest shift in strategy as Republican leaders try to navigate the factions in their caucus, where moderates want to leave the potential for witnesses on the table and conservatives are anxious to quickly acquit President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE.

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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.) said if GOP senators support calling people “who are unhappy about being fired," a reference to Bolton, "then I think the president should get to call his [witnesses] and we should have votes on those.”

“The president gets to call anybody he thinks would be good for his defense, the prosecution can call who they want, but I don’t think we should selectively call witnesses that don't like the president,” Paul said.

Sen. John Kennedy John Neely KennedyMORE (R-La.) stressed that he hasn’t made a decision on calling witnesses but said he expects that if Democrats and moderate Republicans open the door then Trump’s team will be able to call individuals like Hunter Biden and a controversial former Democratic National Committee contractor.

“I assume that if we're going to be fair ... that if we get into having witnesses and evidence that both the prosecution and the defense will be able to weigh in as well,” Kennedy said. “I feel pretty confident, though I don’t know it for a fact, that the defense team is going to want to call its witnesses, including but not limited to the Bidens, [and] as a fact witness the whistleblower.”

The threat, Republicans hope, could help dissuade Democrats, and a handful of their Republican colleagues viewed as swing votes, from agreeing to subpoena Bolton or one of the other witnesses being requested by Democrats.

Senate Minority Leader Charles Schumer Chuck SchumerPelosi orders Capitol flags at half-staff to honor Ginsburg Ginsburg in statement before her death said she wished not to be replaced until next president is sworn in Democrats call for NRA Foundation to be prohibited from receiving donations from federal employees MORE (D-N.Y.) declined to weigh in on Thursday about if he would be willing to engage in a trade where Democrats could call Bolton in exchange for Republicans calling Hunter Biden, who has emerged as a prime fixation for Trump and his allies over his work on the board of a Ukrainian energy company.

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“I’m not going to negotiate out here,” he told reporters. “They haven’t made any offer about any witnesses or any documents.”

The trade off is an idea being floated by conservatives, including Sen. Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death MORE (R-Texas), who is pitching his colleagues on the idea of “witness reciprocity.”

“If they are going to bring witnesses in, we’re not going to do what the House did of a one-sided show trial, and I think it should be at a bare minimum one-for-one,” Cruz told Fox News’s Sean Hannity Sean Patrick Hannity Cruz: Trump should nominate a Supreme Court justice next week Ex-Pence aide: Trump spent 45 minutes of task force meeting 'going off on Tucker Carlson' instead of talking coronavirus Trump ABC town hall pulls in fewer viewers than 'America's Got Talent,' NBA, Fox News MORE. “So if the prosecution brings ... John Bolton, then President Trump can bring a witness. He can bring Hunter Biden.”

Cruz’s idea has garnered attention among conservative media figures including Hannity who told Cruz that he “loved your proposal” and that he hopes Democrats want four witnesses so “I get the four I want”: the Bidens, the whistleblower and House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), who is leading the team of House impeachment managers.

Sen. Kevin Cramer Kevin John CramerOn Paycheck Protection Program, streamlined forgiveness is key McConnell shores up GOP support for coronavirus package Army Corps urges DOJ to settle case with ND over M DAPL damages MORE (R-N.D.) referenced Cruz’s idea, adding that he expects there could be a back-and-forth on the Senate floor to call witnesses.

“I suspect we’ll have one witness at a time that we’ll vote on. And Ted of course has floated the idea of witness reciprocity. Whether it’s one-for-one, or two-for-two or four-for-four,” Cramer said.

“I guess I would be surprised if there weren’t witnesses,” he added, while noting his preference would be that there are no witnesses.

McConnell has maneuvered behind the scenes to try to keep his caucus united as the impeachment trial begins, including closed-door meetings recently with moderates like Sens. Susan Collins Susan Margaret CollinsSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-Maine) and Lisa Murkowski Lisa Ann MurkowskiSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (R-Alaska), while also hearing pitches from Paul and Cruz.

Sen. Roy Blunt Roy Dean BluntCDC tells Congress it urgently needs billion for vaccine distribution On The Money: Trump undercuts GOP, calls for bigger COVID-19 relief package | Communities of color hit hardest financially by COVID-19 | Businesses, states pass on Trump payroll tax deferral Trump undercuts GOP, calls for bigger COVID-19 relief package MORE (R-Mo.), the No. 4 GOP senator, credited McConnell with getting Republicans in line behind the strategy of punting a decision on witnesses until after the trial begins and recognizing that various Republicans have different considerations than other GOP colleagues.

“I think he’s done a good job listening and trying to understand that everybody has their own unique set of considerations here as to how they move forward,” Blunt said. “By doing that [he] has all of us in the same place on the rules that we’ll vote on next Tuesday. That’s a good way to start.”

The fight over impeachment witnesses is expected to come to a head in roughly two weeks. Though Democrats are expected to force votes on witnesses and documents next week, the vote to watch will be after the first phase of the trial.

The rules resolution, according to senators who have seen it, includes a built-in vote after opening arguments and questions from senators on whether there needs to be additional witnesses called or documents compelled.

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That was a victory to a group of moderate senators who publicly urged McConnell to include the provision in the rules resolution.

Murkowski acknowledged that if they open the door to witnesses it could turn into an “unstructured” and “somewhat chaotic,” but that the concession would help put a “solid structure” in place.

“I understand that this process will set that up for later on, and that’s why I think it’s so important that we have some structure. That we have given the courtesy for both sides to present their case, that we have allowed the senators to ask questions ... before we go into this back-and-forth,” Murkowski said.

Though only Sen. Mitt Romney Willard (Mitt) Mitt RomneySenate Republicans face tough decision on replacing Ginsburg McConnell says Trump nominee to replace Ginsburg will get Senate vote GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Utah) has specifically said that he wants to hear from Bolton, a handful of other Republican senators have specifically said they want to leave the door open and make a decision on witnesses after phase one.

Collins in a statement on Thursday evening said that she was “likely” to support calling witnesses after the initial phase of the trial but stressed that she had not made a decision yet on any particular individual.

Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderChamber of Commerce endorses McSally for reelection Trump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures MORE (R-Tenn.), a retiring senator who is viewed as close to McConnell, told the Chattanooga Times Free Press that he was open to additional witnesses and that he had wanted “guaranteed the right to vote” of if more “evidence” is needed after the initial phase of the trial.

"Evidence could be witnesses,” he said. “It could be documents.”