The slow start to the trial had some senators inside the chamber visibly struggling to remain engaged with President Trump’s lawyers.

Justice delayed is justice denied,” Ken Starr intoned.

Senator Tom Cotton, Republican of Arkansas, rubbed his eyes with his right hand and yawned.

Across the aisle, Senator Kirsten Gillibrand, Democrat of New York, grabbed a notebook, stood up from her desk and leaned against a railing in the back of the chamber. Senator Bernie Sanders, the Vermont independent who would almost certainly prefer to be on the presidential campaign trail, leaned back in his chair and clasped his hands over his chest, looking mildly exasperated.

Senator Bob Casey, Democrat of Pennsylvania, appeared to hand over a candy or mint to his desk neighbor, Senator Amy Klobuchar, Democrat of Minnesota, who accepted.

Not all seemed bored by the monotonous proceedings. Of the four closely watched moderate Republicans who could join Democrats to force the Senate to pursue new testimony and documents, no one scribbled more notes than Senator Susan Collins of Maine. Seated next to her, Senator Lisa Murkowski of Alaska stared intently at the lawyers, rocking her chair gently back and forth.

One line seemed to warrant special attention from Republican senators: When Mr. Starr declared that “the commission of a crime is by no means sufficient to warrant the removal of our duly elected president,” suggesting that any crime needed to be serious enough to rise to the level of impeachment. Several senators, including Ms. Collins, seemed to jot it down.