Mr. Kelly, speaking to an audience in Florida, said that he believed the revelations in the book and that the Senate should call witnesses in the impeachment trial. Mr. Kelly and Mr. Bolton overlapped at the White House during much of 2018.

Mr. Bolton has said he would comply with a Senate subpoena to testify during the trial.

Next up: The senators get to ask the questions.

With the close of oral arguments, senators now have the opportunity to ask questions — but only in writing. Starting Wednesday, senators will submit written questions for the House managers and the president’s defense team through Chief Justice John G. Roberts Jr., who is presiding over the trial. The chief justice will read the questions to the lawyers, alternating between submissions from Democrats and Republicans for up to 16 hours over a course of two days.

Senators are expected to vote on whether to hear new witnesses on Friday. The Democrats would need to secure at least four Republican defectors to approve hearing new testimony.

When the session ended on Tuesday, lawyers from the prosecution and the defense shook hands, as Democratic and Republican senators, in an unusual display of harmony, breathed a sigh of relief that they could once again talk in their own chamber.

After defense team wrapped up, Republicans huddled on witnesses.

As soon as Mr. Trump’s legal team finished its oral arguments against removing the president from office, Republicans raced to a room near the Senate floor to decide whether to call witnesses — a step that most of them hope to avoid — but reached no consensus.

The move could prolong the trial and muddle what until recently had seemed to be a smooth march to a speedy acquittal of the president.

During the meeting, Mr. McConnell was said to have held a “whip count” of yes, nos and maybe votes, taking stock of his colleagues’ positions. Mr. McConnell told those in the room that, by his count, he did not yet have enough votes to block witnesses.