“Every senator will have some trouble — we are not, by nature, silent,” said Senator Roy Blunt, Republican of Missouri and the chairman of the Senate Rules Committee. “The desire to hear the sound of your own voice will be frustrated by that rule.”

To remind them, sessions of the trial will begin each afternoon with the Senate sergeant-at-arms intoning the same dramatic command uttered in 1868 at the nation’s first presidential impeachment trial: “All persons are commanded to keep silence, on pain of imprisonment.” (There is no record of a Senate forcing jail time on one of its own.)

It is a harsh reality for a typically talkative group. Matters of grave historical importance aside, they are unaccustomed to limitations on their ability to chat with colleagues, peruse the selections in the designated candy desk (carefully chosen by Senator Patrick J. Toomey, Republican of Pennsylvania) or, in the words of Senator Pat Roberts of Kansas, noisily “mill around like cattle” in the chamber’s well.

On Tuesday, the first formal day of the trial, the rule was already starting to chafe. Senators began to resemble high school students waiting for the bell. Notes were subtly passed between desks. Fingernails were examined, and then bitten.

Senator David Perdue, Republican of Georgia, used a binder to hide a whispered aside to Senator Ted Cruz, Republican of Texas, seated at the neighboring desk. Senator John Thune of South Dakota, the No. 2 Republican, appeared to sneak a glance at the smart watch on his wrist. Senator Bernie Sanders, the Vermont independent seeking the Democratic nomination for president, folded a piece of paper torn from his notepad. Senator Kirsten Gillibrand, Democrat of New York, stifled a yawn as she jotted notes. And about four hours into the trial, late Tuesday afternoon, Senator James Risch, Republican of Idaho, appeared to succumb to the hush and briefly fell asleep, slumped slightly in his chair with his head resting on his right hand.