WASHINGTON—Sen. Bernie Sanders speed-walked out of the Capitol and phoned into a campaign rally in Iowa, apologizing for not being there in person. Sen. Lindsey Graham headed home to South Carolina, where a pulled-pork platter awaited him. And President Trump jetted down to Florida.

The Senate impeachment trial has commanded 11 days of attention, cruising along to what should have been a final vote Saturday. Mr. Trump, the script held, would be acquitted by the Republican majority and the political order would resume its frenetic...