DES MOINES — 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. “It’s said,” a narrator declares, “in here your character is revealed.”

“We saw it with President Obama,” the narrator says. “We’re seeing it with President Trump.”

The ad and Mr. Biden’s speech — which is not expected to discuss details of the impeachment trial, but will contrast the candidate with the president — come against a backdrop of mounting tensions between Mr. Biden and Mr. Trump’s allies in Congress. On Monday, Senator Joni Ernst, Republican of Iowa, suggested that the scrutiny of Mr. Biden and his son at the impeachment trial would hurt Mr. Biden at the caucuses.