"We all know Trump has been messing around in Ukraine over there, saying that they want to investigate you. He has no backbone, we know that. But you, on the other hand, sent your son over there to get a job and work for a gas company that he had no experience with gas or nothing in order to get access for the president. So you're selling access to the president just like he was," the man told Biden, after saying Biden was too old to be president.
"You're a damn liar, man. That's not true. No one has ever said that," a heated Biden responded. "No one has said my son has done anything wrong, and I did not on any occasion, and nobody has ever said it."
Biden also challenged the man to do push-ups and take an IQ test with him to prove his physical and mental fitness to be president.
After the man, who refused to give his name to ABC News, said he is not voting for Biden, the presidential contender and front-runner said, "Of course you're not. You're too old to vote for me."
The man, who identified himself as an 83-year old retired Iowa farmer, told reporters in a press gaggle after the event that he was "in favor of any other Democratic candidate who’s running," adding that Biden is "the last one on my list."
"I’d still vote for a Democrat if he’s the nominee. I would not vote for a Republican," he said.
At the heart of the man's allegations against Biden and his son, Hunter, is that the younger Biden accepted a lucrative seat on the board of directors for Burisma, the Ukrainian company in 2014, during the elder Biden's tenure as vice president.
While the Bidens have not been accused of doing anything illegal, ethics experts say Hunter Biden’s foreign business activity presents ethical concerns.
Trump and his allies have sought to advance a theory that Biden used his position to shield his son and Burisma from investigation by pushing Ukraine's government to fire its then-prosecutor general, Viktor Shokin. But Shokin was widely considered ineffective in dealing with corruption both within Ukraine and internationally, and no evidence has emerged to support that theory. No evidence has surfaced, either, that Biden "sent his son" or otherwise used his influence to get his son a seat on the company's board.
Speaking with reporters after the event wrapped, Biden dismissed questions about whether he had lost his temper.
"I didn’t lose my temper, what I wanted to do was shut this down," he said. "You saw the reaction of all the people here...what I wanted to make clear to him was: if he gets more out of control, this is not appropriate behavior at all. That was the message."
This is not the first time Biden, who has previously launched unsuccessful bids for nation's highest office in 1988 and 2008, engaged in a tense exchange with an attendee at a campaign event.
During Biden's first run for president in 1987, a New Hampshire voter, Frank Fahey, found himself on the receiving end of a tirade from the former Delaware senator after he asked what he thought was an innocent question about Biden's law school experience.
Biden took the question as an insult and partially responded, "I think I probably have a much higher IQ than you do, I suspect. I went to law school on a full academic scholarship."
Last month, Fahey, who is now supporting Biden for president and called him the best candidate for the job, reminded Biden of the exchange in early November in New London, and asked Biden what he would have done differently in his handling of the Anita Hill hearings during Clarence Thomas' Supreme Court confirmation hearing.
"You ask all the easy questions...Frankie, baby, it’s no wonder I love you," Biden joked initially before saying he did all he could to help Hill under the rules of the committee at the time, but admitted that she was treated unfairly.
ABC News' John Verhovek and Molly Nagle contributed to this report.