The Constitution says the chief justice is to preside at the impeachment trial of a president. But what about an ex-president?
Like so much else about the Constitution, the answer is subject to interpretation.
If President Donald Trump's trial begins after January 20, it's not clear whether Chief Justice John Roberts would make his way to the Senate chamber as he did last year for Trump's first trial.
Impeachment scholars, law professors and political scientists offer differing views.
The choices appear to be Roberts, Kamala Harris, who by then will be vice president, or Senator Patrick Leahy from the District of Vermont, who will be the Senate's president pro tem once the Democrats gain control of the Senate.
The issue is "unsettled, completely without precedent, and unspecific in existing Senate rules and precedents", Princeton University political scientist Keith Whittington wrote in an email.
One reason that the Constitution specifies the chief justice to run the president's trial is that the person who otherwise presides over the Senate is the vice president — the very person who would assume the presidency if the chief executive is convicted. That's a bit unseemly.
But, if the stakes are changed and the sitting vice president no longer stands to get the top job, why not have Harris, who by then will have taken over for Mike Pence, preside?
Whittington said he thinks that could happen, "as with the impeachment of any officer other than the president". But he said he "can imagine that the Senate might go the other way and treat a former president the same as a sitting president".
University of Texas law professor Steven Vladeck said the chief justice is the better choice.
The House on Wednesday impeached the president, not the former president, Vladeck wrote on Twitter.
"Indeed, if Trump resigned (or his term ended) mid-trial, it would be more than a little odd for the Chief Justice to give way to the Vice President. The question should be whether the impeached officer was President at the time of impeachment. Here, he was, so Roberts presides," Vladeck wrote.
Another factor in favour of Roberts is that "a trial of a President (even a former President) is a momentous event and having the Chief Justice preside seems more congruent with, or more fitting of, the occasion", Georgia State University law professor Neil Kinkopf wrote in an email.
If it's not Roberts or Harris, who may wish to avoid the appearance of a conflict that presiding over Trump's trial might inflame, the next choice would be Leahy, the senior Democrat in the Senate, Norm Eisen said on CNN. Eisen was a legal adviser to Democrats during Trump's first impeachment.