WASHINGTON — As President Trump’s Senate impeachment trial stretched into its third day on Thursday, the Democratic prosecutors arguing the case for his removal played a video clip from 1999 of Senator Lindsey Graham, now one of Mr. Trump’s staunchest defenders, contradicting a central tenet of the president’s legal defense.
Senators craned their necks from their seats to catch a glimpse of Mr. Graham’s reaction to having his words used against him, but he was not at his desk or indeed in the chamber at all, and his seatmate was left patting his empty chair.
The absence of Mr. Graham, Republican of South Carolina, illustrated an emerging reality for senators as they attend a constitutional proceeding with heavy political consequences: Even when you are one of 100 people weighing the fate of a president in only the third such trial in the nation’s history, restlessness can set in.
“We’re doing our best,” said Senator Kevin Cramer, Republican of North Dakota. “With each break and with each disruption if you will, to the sitting in the chair, there becomes a little less discipline.”
Despite impeachment trial rules that require them to sit silently at their desks when the proceeding is in session, senators have increasingly been wandering out for short or long breaks, to accommodate bathroom stops, telephone calls and even cable television appearances. On Thursday afternoon, as the Democratic impeachment managers took turns speaking during their second full day of presentations, at least 19 senators could briefly be seen out of their seats.
Some rose from their chairs and crossed the chamber to whisper to one another, while others exited the chamber entirely for 15- to 20-minute stretches and could be seen in the cloakrooms on their phones.
As Senator Dianne Feinstein, 86, of California, left the trial on Wednesday almost an hour before the session ended, she bade “good night” to reporters gathered in the hallways. (A spokesman for Ms. Feinstein said she had not been feeling well.)
The day before, Senator Marsha Blackburn, Republican of Tennessee, appeared on Fox News, while her colleagues sat inside the chamber listening to a tense debate between the House impeachment managers and Mr. Trump’s defense team.
Those who remain in their seats have taken up some elaborate distractions. Senator Rand Paul, Republican of Kentucky, who was noodling over a crossword puzzle on Wednesday, could be seen on Thursday laboring over a detailed sketch of the Capitol. Senator John Cornyn, Republican of Texas, twice appeared to swipe at an Apple watch on his wrist, despite the ban on electronics. Multiple senators, in an apparent attempt to rouse themselves, could be seen vigorously chomping on gum. During the lunch break on Thursday, Senator Richard M. Burr, Republican of North Carolina, passed out fidget spinners for his 52 Republican colleagues, the better to pass the time.
Occasionally, the atmosphere has resembled that of an elementary school classroom, in ways large and small.
Each morning, the sergeant-at-arms admonishes senators “on pain of imprisonment” not to speak during the trial. They stash their phones and other electronic devices each afternoon in specially designed cubbies.
A precedent set in 1966 by former Senator Everett McKinley Dirksen, who grew thirsty while delivering a lengthy speech, allows senators to sip milk on the floor, a privilege several Republicans, including Senator Tom Cotton of Arkansas, have invoked during the trial.
They sit at their desks, outfitted with yellow No. 2 pencils, and surreptitiously pass notes and exchange whispers. On Thursday, a pair of senators even appeared to drift into nap time.
“It’s like sitting in the front of the class or sitting in the front pew at church,” Senator Lisa Murkowski, Republican of Alaska, told reporters. “You’ve got to pay attention.”
Most senators have been generally attentive and compliant with the rules, diligently jotting down notes and stretching their legs by standing solemnly behind their desks.
“Other than the occasional bathroom break or going into the cloakroom to get a snack or check your texts or emails or something,” Mr. Cramer said, “everyone’s been pretty well behaved.”
Still, the ambient movement in the chamber on Wednesday afternoon during a particularly antsy stretch threw off one of the impeachment managers.
“I do see the members moving and taking a break,” Representative Jason Crow, Democrat of Colorado, said, interrupting his speech. “I probably have another 15 minutes.”
Republicans said they were having a particularly tough time remaining focused on an argument they call repetitive.
Senator Ron Johnson, Republican of Wisconsin and an ally of Mr. Trump, said the presentations had left him “bored.”
“I don’t know what else they can keep talking about,” Mr. Johnson said. “I mean, we heard their case in the first 13 hours.”
Senator Mike Rounds, Republican of South Dakota, grew exasperated with Representative Adam B. Schiff, Democrat of California and the lead impeachment manager who has spoken more than any of the other six prosecutors. Mr. Schiff, he said, “is now probably in the seventh hour of repeating the first hour, which is frustrating for us. That means a lot more people appear to not be paying attention.”
A bipartisan consensus, however, has emerged on the sanctity of taking breaks during the trial. Senators flock to the bathroom, back to their phones, and more crucially, to the microphones and cameras, where they can air their grievances and opinions to the throngs of reporters waiting for them.
The microphones have become so coveted — especially for Democrats running for president, like Senators Amy Klobuchar of Minnesota and Michael Bennet of Colorado — that lawmakers often must wait in line to snag a few minutes in the spotlight.
Some have also used the breaks to confer with the legal teams on both sides. Senator Roy Blunt, Republican of Missouri, could be seen conversing on Thursday afternoon with Pat A. Cipollone, the White House counsel.
But after the intermission was over, senators obediently returned to their seats, watching a series of videos retracing witness testimony from the House impeachment inquiry.
“I have been pleasantly surprised by the number of people who are there and paying close attention,” said Senator Christopher S. Murphy, Democrat of Connecticut. “I feel like especially during the video, folks are pretty locked in. My suspicion is some of these Republicans have not seen these clips before.”
Peter Baker and Nicholas Fandos contributed reporting.