NEW HAMPTON, Iowa — Former Vice President Joseph R. Biden Jr. on Thursday angrily lashed out at a voter who questioned his son’s overseas business dealings at a campaign stop in Iowa, calling the man a “damn liar” in an unusually heated exchange.
The man — who declined to identify himself to reporters — falsely claimed that Mr. Biden had “sent” his son to work in Ukraine and accused him of “selling access to the president.”
“You’re a damn liar, man,” Mr. Biden shot back. “That’s not true. And no one has ever said that. No one has proved that.”
President Trump faces the possibility of impeachment after pressuring the Ukrainian government to investigate the Bidens. He has made debunked claims about corruption, and there is no evidence that the Bidens engaged in wrongdoing. But Hunter Biden, Mr. Biden’s son, did hold a lucrative position on the board of a Ukrainian energy company. The man said that the younger Mr. Biden “had no experience” in that field.
Before raising that issue, he said Mr. Biden was “too old for the job” of president.
“I’m not sedentary,” Mr. Biden, 77, said. “The reason I’m running is because I’ve been around a long time and I know more than most people know. And I can get things done. That’s why I’m running.”
He went on to encourage the man to do push-ups or go running with him, or take an I.Q. test with him, as the room applauded. At another point, he appeared to say, “Look, fat, look, here’s the deal.” Symone D. Sanders, a senior adviser to Mr. Biden, disputed, in a tweet, that the former vice president had referenced the voter’s weight.
The man eventually told Mr. Biden he would not be voting for him. “Well, I knew you weren’t, man,” Mr. Biden responded. “You think I thought you’d stand up and vote for me? You’re too old to vote for me.”
Afterward, the man said he was an 83-year-old retired farmer, but he declined to give his name to reporters. He said Senator Elizabeth Warren of Massachusetts was his top choice in the Democratic primary race.
The moment captured Mr. Biden appearing to lose his cool when questioned about his son — a topic Mr. Trump is sure to press him on if he faces Mr. Biden in a general election. But Mr. Biden’s forceful pushback also comes as some Democratic voters say they would like to see the former vice president more aggressively defend himself and his family.
Speaking to reporters after the event, Mr. Biden said he wanted the focus to remain on Mr. Trump and his wrongdoing, and he predicted that the general election was “even going to be meaner.”
“I have overwhelming respect and love for my son, and I find myself occasionally getting frustrated” over assertions “that are simply not accurate,” he said. “But as my son would say, ‘Dad, just keep your cool. Just don’t let it get to you.’”
When a reporter asked about “losing your temper,” Mr. Biden denied that he had. “You want to see my temper, keep going,” he said, prompting some laughter. “What I wanted to do is shut this down.”
The exchange instantly overshadowed the campaign’s announcement that former Secretary of State John F. Kerry was endorsing Mr. Biden.
Still, the Kerry endorsement is among Mr. Biden’s most significant to date. His support provides Mr. Biden the backing of the Democratic Party’s 2004 presidential nominee and a past winner of the Iowa caucuses.
“I believe Joe Biden is the president our country desperately needs right now, not because I’ve known Joe so long, but because I know Joe so well,” Mr. Kerry said in a statement. “I’ve never before seen the world more in need of someone who on Day 1 can begin the incredibly hard work of putting back together the world Donald Trump has smashed apart.”
The Biden campaign announced Mr. Kerry’s endorsement as Mr. Biden began the sixth day of his eight-day “No Malarkey” bus tour in Iowa, which brought him to New Hampton for an event on Thursday morning. The bus tour and endorsement come just two months before the caucuses in Iowa, where Mr. Biden is scrambling to regain traction after months of slipping in the polls, amid organizational challenges and struggles to generate enthusiasm.
Mr. Kerry will join Mr. Biden in Iowa on Friday and will also campaign with him in New Hampshire on Sunday, the Biden campaign said.
In his own presidential contest, Mr. Kerry, like Mr. Biden, was perceived as a more establishment, centrist figure, and he struggled both nationally and in the early-voting states in the months leading up to the Iowa caucuses. But following a major campaign push in the state, he went on to win there with a message focused on electability, and eventually to become the nominee.
In his endorsement, Mr. Kerry cited Mr. Biden’s legislative accomplishments, and in particular, his work on foreign policy as vice president and as a former chairman of the Senate Foreign Relations Committee.
“He’s the candidate with the wisdom and standing to fix what Trump has broken, to restore our place in the world, and improve the lives of working people here at home,” Mr. Kerry said.
Those comments come as Mr. Biden is increasingly highlighting his international experience as a way to contrast himself both obliquely with his Democratic rivals, and more overtly with Mr. Trump. His team on Wednesday released a video showing footage of Mr. Trump being mocked by world leaders at a NATO meeting in London and concluding, “We need a leader the world respects.” By Thursday afternoon, it had amassed more than nine million views on Twitter.
Thomas Kaplan reported from New Hampton, and Katie Glueck from New York.