Senate Democrats are pressing Chief Justice John Roberts to rule in favor of calling witnesses at 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's impeachment trial, while Republicans argue it could force his recusal from potential Supreme Court cases.

Democrats say it's simple: A trial can’t be a fair one without witnesses. Republicans counter that if Roberts rules on witnesses, he will have to recuse himself from any Supreme Court case on Trump's claims of executive privilege over potential witnesses like 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.

“I don’t know how you have a serious trial unless you hear from witnesses who know in fact what the facts are, what happened,” said Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.), a leading candidate for the Democratic presidential nomination.

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“I think it would be appropriate for the chief justice to do what I think should be done, and that is to allow witnesses to testify,” he added.

While Roberts is expected to refer major disputes over trial procedures to the entire Senate for a vote, some Democrats hope he will make his own rulings on what they say are basic questions of fair jurisprudence.

Democrats argue that holding a trial but blocking the consideration of relevant witness testimony and document review would fall well short of what’s considered a fair trial in any court of law.

“We’ve been working to get an agreement with Republicans about the relevant witnesses for the trial and relevant documents and we’re continuing that effort,” said Sen. Chris Van Hollen Christopher (Chris) Van HollenMid-Atlantic states sue EPA over Chesapeake Bay pollution Trump payroll-tax deferral for federal workers sparks backlash Senators urge administration to make payroll tax deferral optional for federal workers MORE (D-Md.). “Those types of questions will have to be considered as the trial proceeds.”

Senators who want Roberts to intervene have pointed to the precedent set by then-Chief Justice Salmon P. Chase in the 1868 impeachment trial of President Andrew Johnson.

Chase broke two ties on procedural deadlocks, both votes on motions to adjourn.

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The Senate voted 27 to 21 to defeat an amendment proposed by Sen. Charles Sumner (R-Mass.) stating the chief justice had no authority to vote to break ties.

Senate Democratic 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.) has focused his demands on four witnesses: Bolton, Mulvaney, senior White House adviser Robert Blair, and senior Office of Management and Budget official Michael Duffey.

Republicans are pushing back, arguing Trump is expected to assert executive privilege to block the four witnesses Democrats have requested.

“The chief justice is supposed to preside, not make decisions for the Senate,” said a senior Senate GOP aide.

“It would be a conundrum for the chief justice,” the aide added. “Let’s say [Roberts] says, ‘I’m going to break the tie,’ and John Bolton has to testify. Wouldn’t he have to recuse himself when the Supreme Court gets the question as to whether executive privilege applies?”

The aide said it “would be an unprecedented interference of the third branch into the first branch.”

A second senior Senate Republican aide said “the chief justice doesn’t vote,” adding that “you need 51 votes” to decide trial procedures.

Sen. 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) told The Hill she has received briefing materials from former senators who participated in the 1999 Clinton trial arguing Roberts should not be able to break a tie.

So far, she is still thinking about what his role should be.

“It’s my understanding that if it’s a tie, it fails,” she said, summarizing the briefing paper.

The chief justice is expected to do everything possible to avoid politicizing his or her role through partisan rulings, but Roberts could also come under harsh criticism from Democrats if he takes no action and allows Republicans to proceed with their preferred format, which does not include witnesses.

When Roberts swore in senators at the start of the trial on Thursday, he asked them to “do impartial justice according to the Constitution and laws.”

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“Every Senate impeachment trial in our history, all 15 that were brought to completion, featured witnesses. Every single one,” Schumer said Thursday. “The precedent in impeachment trials in the Senate is to have witnesses. To have no witnesses would be a dramatic break with precedent.”

Schumer is also seeking to subpoena three sets of documents. He wants memos and emails related to efforts to pressure Ukraine to announce certain political investigations; documents related to the decision at the White House to withhold a meeting with the newly elected Ukrainian president; and communication touching the decision to withhold $391 million in military assistance to Ukraine.

So far, two Republican senators have said they will likely vote to call additional witnesses.

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) told reporters, “I presume I’ll be voting in favor of hearing from John Bolton, perhaps among others.”

Sen. 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) issued a statement Thursday saying “it is likely that I would support a motion to call witnesses” after both sides present opening arguments.

A third Republican, Murkowski, said she is “curious” to hear what Bolton has to say but won’t reveal how she’ll vote on witness subpoenas.

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Finding a fourth GOP senator to vote for subpoenas would be a tougher task, raising the prospect of a Senate deadlock.

Republicans hold a 53-47 majority in the chamber.

Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (Calif.), the top-ranking Democrat on the Judiciary Committee, said Roberts should break a 50-50 tie on a major procedural question, such as whether to subpoena Bolton or Mulvaney.

“It seems to me it should,” she said. “Tie vote is essentially a no on whatever the issue is. I’m not sure in this kind of situation that the body wants to sustain immobility, that you can’t move out of it. So something ought to break a tie.”

But Sen. Richard Shelby Richard Craig ShelbySenate GOP eyes early exit Dems discussing government funding bill into February GOP short of votes on Trump's controversial Fed pick MORE (R-Ala.) said it would be odd for the chief justice to decide procedural questions that have become very politicized in recent weeks.

“This is a Senate procedure,” he said. “In a committee, if you have a tie vote you don’t prevail. And that’s what would happen here, I would think.”

Correction: This article was updated Jan. 23 to reflect that Chief Justice Salmon P. Chase voted on two motions to adjourn the Senate. An amendment to prevent the chief justice from voting was soundly defeated without Chase’s participation.