WASHINGTON — Hours into the first long night of President Trump’s impeachment trial on Tuesday, Senator Bernie Sanders of Vermont kept checking his watch. Senator Elizabeth Warren of Massachusetts sipped hot water — one of two drinks allowed in the chamber — to warm up. Senator Amy Klobuchar of Minnesota jotted down notes. And Senator Michael Bennet of Colorado stood and listened.

Around the same time, more than 1,000 miles west at a community college in Fort Dodge, Iowa, former Vice President Joseph R. Biden Jr. was warning an audience about the divided nation that the next president would inherit.

And at a veterans’ hall in Cedar Rapids, Iowa, former Mayor Pete Buttigieg of South Bend, Ind., was coming out swinging against Mr. Trump, prompting 1,200 voters to chant “Pete! Pete!”

“Wouldn’t it be nice to put the tweets behind us?” Mr. Buttigieg said.

Something extraordinary is happening to the Democratic presidential primary: An intensely competitive race has been thrown into a state of semi-suspended animation less than two weeks before caucusing begins. Three candidates who have a shot at breakout performances in Iowa on Feb. 3, Ms. Warren, Mr. Sanders and Ms. Klobuchar, are suddenly stuck at the Senate impeachment trial in Washington, while their rivals have the campaign trail largely to themselves.