The defining battle in President Donald Trump's impeachment trial concerns whether the Senate will hear from key witnesses, including some of Trump's closest advisers.

Senate Democrats have said they want four key witnesses, including former national security adviser John Bolton, and White House Chief of Staff Mick Mulvaney, to testify in the trial.

But it's unclear whether Democrats can convince at least four Republican senators to defect from their party and vote to include witnesses.

Some legal experts are calling on Chief Justice John Roberts, who's presiding over the Senate trial, to issue subpoenas himself if the Senate refuses to.

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The defining battle in President Donald Trump's impeachment trial concerns whether the Senate will hear from key witnesses, including some of Trump's closest advisers.

On Sunday, the dynamics of that fight changed. The New York Times reported that Trump's former national security adviser John Bolton wrote in his forthcoming book that Trump told him last year he would withhold military aid to Ukraine until the Ukrainian president acceded to his demands for investigations into his political rivals.

The bombshell report has undermined Trump's impeachment defense and intensified pressure on GOP senators to call Bolton to testify, something he's already said he'd do if subpoenaed.

It's also provoked some legal experts to call on Chief Justice John Roberts, who's presiding over the Senate trial, to issue subpoenas himself if the Senate refuses to.

Neal Katyal, the former solicitor general under President Barack Obama, Georgetown Law School professor Joshua Geltzer, and former GOP congressman Mickey Edwards, argued in a Monday New York Times op-ed that, under the 1868 Senate impeachment rules, Roberts has full authority to subpoena witnesses and documents in the impeachment trial.

"The Democrats' impeachment managers should immediately ask the chief justice to issue subpoenas for key witnesses and documents, insisting that the Senate rules make him and him alone the decision maker about whether to 'make and enforce' those subpoenas," they wrote.

While they conceded that the vote of a majority of senators is necessary to decide evidence questions, including whether to compel a witness to answer a specific question or whether to enter a document into the record, the lawyers argued it's Roberts' "prerogative" and "responsibility" to ensure the evidence is debated by the Senate.

Senate Democrats want at least four key witnesses, including Bolton and acting White House chief of staff Mick Mulvaney, to testify. Republicans, led by Senate Majority Leader Mitch McConnell, have so far remained staunchly opposed to calling witnesses.

CNN legal analyst Elie Honig, a former prosecutor, argued Roberts is facing "a legacy-defining moment of truth."

He tweeted, "Will he aspire to the Rehnquist model of proudly doing nothing, or will he fulfill the Constitution's directive that he "shall preside" in some meaningful way?"

Some Democrats have similarly voiced support for a Roberts intervention.

"Despite the Jury foreman McConnell's best attempt to rig the trial for the defendant, the judge should make sure all the facts are entered into evidence and the witnesses are heard," Jesse Ferguson, a Democratic strategist and former spokesman for Hillary Clinton, told Insider.

Sen. Bernie Sanders, a top 2020 Democrat, weighed in last week, arguing that Roberts should step in to allow witnesses.

Sen. Bernie Sanders. Brian Snyder/Reuters

"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," Sanders said.

Corey Brettschneider, a constitutional-law and politics professor at Brown University, cautioned that asking Roberts to intervene should be a last resort for Democrats. He argued that, before it comes to that, the House should subpoena Bolton first.

"If all other options are exhausted and the only path forward is for the democratic managers to make a request that Chief Justice Roberts subpoena the managers without majority support in the Senate, it is worth trying," he told Insider.

He added, "Defenders of this path have made a strong argument that the rules allow it and we can hope that the Chief Justice cares more about the law and his place in history than about what this president and Republican Senators think of him."

'The bottom line is 51 senators win'

Other legal experts say Roberts doesn't have the authority to overrule a majority of the Senate under Majority Leader Mitch McConnell's rules. And even if Roberts does believe he has such authority, he wouldn't risk politicizing himself by acting on it.

Frank Bowman, a constitutional-law professor at the University of Missouri School of Law, called the argument articulated in Monday's Times op-ed "crazy" and said it's "vanishingly unlikely" that Roberts decides to "step in the middle of this incredible snarling axe fight and pick a side."

"I don't think there's any pressure on Roberts to do anything — I think that's a figment of the frenzied imaginations of the commentariat," Bowman told Insider. "The Constitution says the Senate has the sole power to try impeachments and, regardless of what some old rule may say, the bottom line is 51 senators win."

Republicans insist the senators themselves should determine the rules of the trial.

GOP Sen. Lisa Murkowski, a key moderate who says she's "curious" to hear Bolton's testimony, told The Hill last week that she doesn't believe Roberts has the authority to break a tie in a Senate vote on the rules.

Sen. Lisa Murkowski, R-Alaska. Jacquelyn Martin/AP

Murkowski is one of just a few Republican senators who've expressed openness to admitting new evidence and witnesses into the trial. On Monday, Sens. Mitt Romney and Susan Collins, both Republicans who've been critical of the president, said they're leaning towards voting in favor of calling witnesses.

But Democrats would need at least four Republicans to defect in order to reach the necessary 51-majority vote.

GOP strategist Matt Mackowiak also said Roberts is "unlikely" to overrule a Senate vote on the issue.

"He won't want to insert himself," he told Insider. "He will let the will of the majority of the Senate work, likely on Friday."

Notably, a CNN poll published last week found that a large majority of Americans — 69% — think the Senate should call new witnesses to testify during the trial — a key issue which Democrats strongly favor and most Congressional Republicans reject.

Republican voters are divided on the issue with 48% in favor of calling new witnesses and 44% opposed to hearing new witnesses.