Democrats prepared to accuse Republicans of a cover-up if they do not vote for witnesses pounced on Mr. Sekulow’s remarks about Mr. Bolton, saying that his reference to “unsourced allegations” proved their point that the Senate must subpoena him to clarify his precise account.

“Once again, the president’s team, in a way that only they could, have further made the case for calling John Bolton,” Representative Adam B. Schiff of California, the lead House manager, told reporters during a break in the proceedings.

Proponents of calling Mr. Bolton also got an unexpected bit of support late Monday from John F. Kelly, the former White House chief of staff, who told an audience in Florida that he believed Mr. Bolton’s account and supported the Senate seeking direct witnesses.

“I think some of the conversations seem to me to be very inappropriate, but I wasn’t there,” he said, according to The Sarasota Herald-Tribune. “But there are people that were there that ought to be heard from.”

Republican leaders appeared to be slowing down what had been a breakneck trial schedule to allow for fuller consideration of the matter. They were hopeful that by putting distance between the emergence of Mr. Bolton’s account and the vote on witnesses, tensions would cool enough to hold a majority intact.

“We’re trying to get everyone on the same page,” said Senator John Cornyn, Republican of Texas.

Beginning Wednesday, senators will have up to 16 hours over two days to question the prosecution and defense teams. Much of that time will probably be used to allow the two sides to respond to each other’s arguments, but Democrats and Republicans were also preparing pointed questions intended to highlight soft spots in the respective cases.