Under the chamber’s rules, the decision to allow witnesses into the Senate trial will rely on a simple 51-vote majority of the Senate. This means that for Democrats to win, they need four Republicans to back their demands for witnesses to offset their minority numbers. 

Republican Sens. Mitt Romney of Utah and Susan Collins of Maine are possible GOP swing votes, as they’ve previously mentioned they prefer to have witnesses. However, without certainty over how many Republicans will actually vote in favor of the Democrats’ demands, there’s no way of knowing yet if the Senate will approve witnesses. 

Another option would be to get three Republican members in support and hope that the presiding officer, breaks the resulting 50-50 tie. Usually, the tie breaker in Senate proceedings is the vice president. However, being that this is a special case of an impeachment trial, the responsibility would lie on the chief justice. John Roberts was officially sworn in for the role last Thursday.

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Roberts has yet to announce whether or not he’d be in favor of witnesses, but there is a precedent of calling witnesses into impeachment trials. 

Chief Justice Salmon Chase in 1868 cast the tie-breaking vote in President Andrew Johnson‘s impeachment trial. History, although serving as a precedent for Roberts to possibly follow, doesn’t obligate him to vote in favor of witnesses. The Constitution is also vague on the matter and doesn’t offer a legal ruling. 

There are guesses, however, as to how he will vote. As an institutionalist, his main agenda is guessed to be unbiased. The biggest clue for this assumption comes from his confirmation hearings in 2005, where he referred to justices as baseball umpires who simply call “balls and strikes.”

Either outcome, Chief Justice’s Roberts’ decision will have little effect the outcome of the trial, as 20 Republican Senators are still needed to vote to remove the President with or without witnesses.