Former Vice President Joseph R. Biden Jr., after coming under assault at the impeachment trial, plans on Thursday to offer a forceful rebuke of President Trump that will double as a closing message ahead of the Iowa caucuses on Monday.

Mr. Biden, who faces a close and fluid race in the leadoff caucus state, will deliver a speech in Waukee, a suburb of Des Moines, in an urgent late-stage effort to focus Democrats on the question of which candidate can defeat Mr. Trump. The speech will also serve as a pre-emptive rebuttal to a rally that Mr. Trump is holding in Des Moines later on Thursday, capping a week in which the president’s legal team tried to train attention on Mr. Biden and his son, Hunter Biden, at the impeachment trial.

The Biden campaign will also start airing a new 60-second television ad in Iowa that begins with images of the White House and the Oval Office as a narrator declares, “It’s said in here your character is revealed.”