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A major question looms over President Donald Trump's impeachment trial: Will there be any witnesses?

The decision hinges on a simple 51-vote majority of the Senate under the chamber's rules, meaning the 47 Democratic senators are looking for four Republicans to back their demand that several top current and former Trump administration officials testify.

Republican Sens. Mitt Romney of Utah and Susan Collins of Maine, who are among the handful of possible GOP swing votes, have suggested they favor some witnesses, leaving Washington fixated on whether — as House impeachment manager Hakeem Jeffries, D-N.Y., put it — the Senate will reach "the magic number" of four that could change the course of the trial.

But that's not the only way to get witnesses.

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Alternatively, Democrats could reach the simple majority threshold with just three Republican members if the presiding officer breaks the resulting 50-50 tie. In normal Senate business, that job would fall to Vice President Mike Pence, the president of the Senate. But the rare instance of an impeachment trial is presided over by the chief justice, in this case John Roberts, who was officially sworn in for the role on Thursday.

With at least two GOP senators possibly backing witnesses — or potentially three, including Lisa Murkowski of Alaska, according to The New York Times (citing Alaska Public Media) — the contingent of defecting Republicans could put calling witnesses within reach.

Judges typically rule in favor of allowing relevant witnesses at trials and, although Roberts has offered no clues about how he would approach the situation, there is precedent for it.

In 1868, Chief Justice Salmon Chase cast two tie-breaking votes in President Andrew Johnson's impeachment trial. Indeed, the current Senate rules for impeachment trials include a history section noting, "The chief justice has voted in the case of a tie." (The same section says that in the Johnson trial, the senators also voted to support the chief justice's conclusion that he could cast tie-breaking votes, noting "the Senate turned down each attempt to prevent the chief justice from voting.")

This history provides a path for Roberts, but not a requirement, since there is not a rule mandating him to do so. The Constitution is silent on the matter.

While the entire possibility might seem fairly arcane and hypothetical, there are signs Majority Leader Mitch McConnell, R-Ky., is already gaming out a strategy. His spokesperson recently told a Vox reporter, without elaborating, that "ties lose" in impeachment trial votes.

A spokesperson for Minority Leader Chuck Schumer, D-N.Y., declined to offer a position on the matter.

At the Supreme Court, Roberts has avoided some thorny political questions. Only time will tell if deadlocked votes in the Senate require his hand in Trump's trial.

Later, Roberts addressed the issue during the trail in the Senate.

He said it would be "inappropriate" for him to break any 50-50 tie votes during the Trump impeachment trial. His brief explanation resolved any lingering questions about the issue and appeared to set a precedent against tie-breaking votes for future impeachment proceedings.