Not only could Mr. Biden be forced to be in D.C. at a critical moment in the presidential campaign, but so could many of his chief rivals — the half-dozen senators also vying for Democrats’ presidential nomination, impeachment experts said.

For that matter, if the House chooses to impeach Mr. Trump on charges stemming from the special counsel’s Russia investigation, aides said it could open the door to witnesses such as fired FBI Agent Peter Strzok or even major figures from the Obama administration.

Mr. Trump could even be present for the entire spectacle. Experts said the Senate would have a hard time refusing him if he demanded to confront the witnesses against him.