Senator Patrick Leahy, the longest-serving Democrat, will preside over Trump’s impeachment trial.

Senator Patrick J. Leahy, the Senate president pro tempore, is expected to preside over former President Donald J. Trump’s impeachment trial when it formally begins on Tuesday, assuming a role filled last year by Chief Justice John G. Roberts Jr., aides and other officials said on Monday.

The Constitution states that the chief justice of the United States presides over any impeachment trial of the president or vice president. But it does not explicitly give guidance on who should oversee the proceeding for others, including former presidents, and it appeared that Chief Justice Roberts was uninterested in reprising a time consuming role that would insert him and the Supreme Court directly into the fractious political fight over Mr. Trump.

Mr. Leahy, a Vermont Democrat, recently reclaimed the mantle of president pro tempore — the position reserved for the longest-serving member of the majority party — when Democrats took control of the Senate. Mr. Leahy, 80, has been in office since 1974.

The role was largely ceremonial in the first impeachment trial of Mr. Trump a year ago. But as the presiding officer, Mr. Leahy could issue rulings on key questions around the admissibility of evidence and whether a trial of a former president is even allowed under the Constitution. (Mr. Leahy is also still expected to have a vote in the trial, like other senators.)

The job could also have gone to Vice President Kamala Harris, in her capacity as president of the Senate. But there were clear drawbacks for Ms. Harris in overseeing what is likely to be a divisive proceeding that is all but certain to be regarded by some as an effort by Democrats to use their newfound power to punish the leader of the rival political party.

Mr. Leahy’s presence on the dais could open Democrats to similar charges from the right, particularly if he issues a contentious ruling, but officials said there was no clear alternative without the chief justice. In a statement, Mr. Leahy said he would take “extraordinarily seriously” his trial oath to administer “impartial justice.”

“When I preside over the impeachment trial of former President Donald Trump, I will not waver from my constitutional and sworn obligations to administer the trial with fairness, in accordance with the Constitution and the laws,” he said.

Senate aides suggested Ms. Harris may still be able to intervene to break a tie if the trial ever deadlocked 50-50, as she can in the Senate’s normal course of business.

It would take two-thirds of the Senate, 67 votes, to convict Mr. Trump, but if he were convicted, only a simple majority would be needed to bar him from holding office again.

The chief justice declined through a spokeswoman to comment.