(CNN) Joe Biden grew visibly frustrated with an audience member at a campaign event on Thursday, calling the man a liar and challenging him to a push-up contest when the man asked about his son and said the former vice president was too old.

The audience member, who asked the question during a stop on Biden's Iowa bus tour in New Hampton, accused Biden of "selling access to the President," referring to Biden's son, Hunter, who served on the board of a Ukrainian gas company while his father was vice president. Hunter Biden said recently he used "poor judgment" in serving on the board while his father was pushing anti-corruption measures in Ukraine on behalf of the US government, but added that he didn't do anything improper. There is no evidence of wrongdoing by either Joe or Hunter Biden.

The man accused Biden of sending his "son over there, to get a job and work for a gas company, that he had no experience with gas, nothing."

"You're a damn liar, man, that's not true," Biden responded. "And no one has ever said that, no one has proved that."

After the man pushed back again, Biden replied, "You said I set up my son to work for an oil company, isn't that what you said? Get your words straight, Jack!"

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