Former Vice President Joe Biden had a testy exchange with an Iowa voter on Thursday, calling the man a "damn liar" in a clip that has gone viral on social media.

In the clip, a voter at a Biden town hall who identifies himself as a retired farmer, tells the former vice president he has two problems with him.

"One, is you're damn near as old as I am. You're too old," he tells Biden. "I'm 83 and I know damn well I don't have the mental faculties I did ... 30 years ago."

Then he accuses Biden of sending his son Hunter to Ukraine to work for the gas company Burisma. "So you're selling access to the president, just like [Trump] is," the voter says.

At that point, Biden interjects to angrily rebut the man's accusations.

"You're a damn liar, man," he says of the claims that he arranged Hunter Biden's lucrative appointment to the board of Ukraine energy company Burisma. "You said I set up my son to work at an oil company. Isn't that what you said? Get your words straight, Jack!"

The younger Biden's role at Burisma has come under intense scrutiny, as congressional impeachment hearings center around an investigation into whether President Donald Trump pressured the government of Ukraine to investigate Hunter's job there.

Biden then turns to the voter's insinuation that his age would render him unfit for the presidency -- even challenging the man to do physical exercise alongside him or to compare their mental acumen together.

“I’m not sedentary,” said Biden, 77. "Look, 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. And you want to check my shape, man, let’s do push-ups together here, man, let’s run, let’s do whatever you want to do, let’s take an IQ test."

Although Biden continues to have a healthy lead in national Democratic presidential primary polls, there is some evidence voters are uncomfortable with voting for a candidate his age. An NBC/Wall Street Journal poll in February found 62% of voters either had "reservations" or were "very uncomfortable" voting for a candidate who was older than 75.