How long can the US Senate sit still? Twenty minutes

The 100 US senators who will determine whether to remove President Donald Trump from office at the end of his impeachment trial managed to stay seated and listen to the Democratic case against him for about 20 minutes on Thursday.

Beginning at 1:20 p.m., lawmakers began peeling off slowly to the Republican and Democratic cloakrooms at the back of the Senate chamber, where they can make calls and check their smartphones for emails and text messages.

They began wandering shortly after US Representative Adam Schiff, the lead Democrat presenting the case against Trump, thanked them for their attention through about 20 hours of proceedings over the first two days.

"I'm not sure the Chief Justice is fully aware of just how rare it is, how extraordinary it is, for the House members to be able to command the attention of senators sitting silently for hours, or even for minutes, for that matter," Schiff said, alluding to Chief Justice John Roberts, who is presiding over the hearings.

"Of course, it doesn't hurt that the morning starts out every day with the sergeant at arms warning you that, if you don't, you will be imprisoned."

Despite that warning about the power of Senate security, members of both parties disregarded rules that call for them to remain seated and silent through the impeachment trial before voting as jurors on Trump's fate. The rules also ban cellphones in the chamber and allow only two beverages: water or milk.

Democratic Senator Michael Bennet, one of the chamber's four presidential candidates, was the first lawmaker to spend a few minutes in a cloakroom, where members meet for private discussions. He repeated the trip 25 minutes later. A few minutes after that, Republican Senator Rob Portman headed into the Republican cloakroom on the other end of the back wall.

From about 1:52 p.m. to 2:30 p.m., at least nine Democrats and 22 Republicans left their seats. Some lawmakers were gone for just a few minutes, others nearly half an hour.

Two other Democratic presidential candidates - Bernie Sanders and Amy Klobuchar - also headed to the cloakroom, Sanders for eight minutes and Klobuchar for 16.

At intervals, one Republican senator or another could be seen through cloakroom doors sitting in a cushioned wing-back chair and fingering a cellphone. At one point, voices could be heard from inside.

Trump's impeachment trial poses a challenge for the Senate Democrats seeking their party's presidential nomination, who are stuck in Washington while early voting approaches in Iowa and New Hampshire.

The fourth Democratic presidential candidate, Senator Elizabeth Warren, was not fully visible.

Some Republicans who remained seated had another way to distract themselves, after small toys known as fidget spinners were handed out at a private lunch of party members before the hearing.

"I saw somebody grab up a few of them, so they must have some real anxiety going along with this," said Senator Mike Braun, adding that he hadn't needed one.