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 as a narrator declares, “It’s said in here your character is revealed.”
“We saw it with President Obama,” the narrator says. “We’re seeing it with President Trump.”
The speech and ad 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.
And on Tuesday, Senator Rick Scott, Republican of Florida, released a television ad to be aired in Iowa that baselessly accuses Mr. Biden of corruption, echoing Mr. Trump’s unfounded attacks on the former vice president.
Mr. Biden has spent the past two days pushing back against Ms. Ernst’s remarks, fund-raising off them and releasing a digital ad on Tuesday. At a campaign event in Sioux City, Iowa, on Wednesday, Mr. Biden pointed to Mr. Scott’s ad as further evidence that Republicans were trying to derail his candidacy.
“A senator from Florida, sitting in Washington, has decided to start running negative ads against Joe Biden just days before the Iowa caucus,” Mr. Biden said. “What do you think that’s about?”
He then laughed.
“Look, it’s simple,” he continued. “They’re smearing me to try to stop me, and they know if I’m the nominee, I’m going to beat Donald Trump like a drum.”
The Biden campaign has long hoped that in the final days before the caucuses, Iowans will grow more intensely attuned to arguments about electability — though in recent weeks, Mr. Biden’s own argument on that front has been squeezed, especially by Senator Amy Klobuchar, a moderate Minnesota Democrat, as well as by Senator Bernie Sanders of Vermont, a democratic socialist who has gained traction in the race, as the Biden campaign has privately acknowledged to donors.
In the speech, his campaign said, Mr. Biden plans to remind voters that Mr. Trump must be defeated in order to make progress on issues like health care, gun violence, climate change and how America is viewed on the world stage. And he will speak about the nature of the country, as he has done on the campaign trail this week in Iowa, where he has repeatedly declared that “character is on the ballot.”
Thomas Kaplan reported from Des Moines, and Katie Glueck from Sioux City, Iowa. Nick Corasaniti contributed reporting from Des Moines.