Chief Justice John G. Roberts Jr. inserted himself into the impeachment trial Thursday, refusing to read a question from Sen. Rand Paul that apparently trod too closely to the identity of the whistleblower who ignited the entire affair.
Other than a scolding or two for breaking decorum, the chief justice had been a disinterested referee in the trial of President Trump, but his refusal to read Mr. Paul’s question put him in the middle of the biggest political fight of this century.
“The presiding officer declines to read the question as submitted,” the chief justice told Mr. Paul.
He did not explain his reasoning, but Mr. Paul later indicated it was because the chief justice is trying to protect the identity of the whistleblower whose complaint last summer triggered the impeachment effort against Mr. Trump.
Perhaps bolstered by his ruling, Democrats said they want to see Chief Justice Roberts play an even bigger role, asking him to decide all major rulings on witnesses, evidence and arguments over privileged material, should the trial get to that point.
“The chief justice could decide,” said Rep. Adam B. Schiff, Democrats’ impeachment maestro.
But the president’s legal team rejected that idea, saying it would cede the Senate’s powers to set its own rules and make its own judgments to another branch of government.
The Constitution gives the sole power to try impeachments to the Senate, with the chief justice assigned a presiding role. What that means, though, is not set in stone.
The chief justice could face another key moment Friday when the Senate is expected to vote on whether to call more witnesses and pursue documents. All 47 Democrats are expected to back that motion, and as many as four Republicans are pondering whether to side with them.
If exactly three of them do vote for witnesses, the motion could end in a 50-50 tie. Under usual circumstances the vice president, acting as presiding officer, could cast a tie-breaking vote. But it’s not clear whether the chief justice could play that role.
Without a tie-breaker, a 50-50 vote would fail, since it didn’t achieve a majority.
Senators say the rules could be read either way on the chief justice’s ability to cast a vote.
But Sen. Josh Hawley, Missouri Republican, suggested Thursday the chief justice would be inserting himself too deeply into the affair if he did try to vote.
“Not casting a tie-breaking vote is him just staying out of it,” the senator told reporters at the Capitol.
Sen. Mike Braun, Indiana Republican, said the best approach is to avoid a 50-50 tie, calling it “uncharted territory.”
“There is not real certainty about what power the presiding officer has and I do think, everything we understand is that if you don’t like the decision, 51 votes can overrule the chair,” the senator said.
Mr. Paul said he could have taken that path and tried to challenge Chief Justice Roberts’ ruling on his question, but he didn’t want to prolong the trial.
He did, though, defend his question.
“I can tell you my question made no reference to any whistleblower,” he said. “I don’t know who the whistleblower is.”
On Twitter earlier in the day he said his question was “about whether or not individuals who were holdovers from the Obama National Security Council and Democrat partisans conspired with Schiff staffers to plot impeaching the president before there were formal House impeachment proceedings.”
Some of his fellow Republicans said Mr. Paul should have been allowed to ask about the whistleblower, who in their minds is at the center of the case against Mr. Trump. They questioned the chief justice’s intervention.
“That’s concerning,” said Sen. Joni Ernst, Iowa Republican. “I think that all questions need to be considered by the body and hear those answers. And if a senator is presenting a question, I think we should be able to hear the answer to that.”
The whistleblower’s identity is widely discussed in political circles, though The Washington Times is not publishing the name since it has not been confirmed.
There is a theory in the GOP that the person is a CIA analyst with ties to the Democratic Party and former Vice President Joseph R. Biden, and may have been involved when Mr. Biden was pressuring the Ukraine government to fire its chief prosecutor.
Democratic impeachment managers insist Mr. Biden’s actions were on the up-and-up, and that the former vice president doesn’t need to be part of the trial against Mr. Trump.
And they have rebuffed GOP efforts to talk about the whistleblower, saying it would be illegal and unethical to reveal the person.
During Thursday’s proceedings Sen. Elizabeth Warren, a Massachusetts Democrat who is running for the party’s nomination to unseat Mr. Trump, posed the question of whether Chief Justice Roberts’ role overseeing a trial without any witnesses would damage his legitimacy and that of the Supreme Court.
Mr. Schiff didn’t take the bait.
“I think the chief justice has presided admirably,” he said.
• Gabriella Muñoz contributed to this article.