"Hunter, you know nothing about energy, you know nothing about China, you know nothing about anything, frankly. Hunter, you're a loser," Trump said.
Trump also proposed a new T-shirt to be sold by his campaign that would read, "Where's Hunter?" That was an apparent reference to Hunter Biden's lack of public appearances as his business dealings come under greater scrutiny.
"Let's do another t-shirt: Where's Hunter?" Trump said, prompting applause from the crowd.
Trump also attacked the elder Biden, telling Hunter that his father wasn't a good politician.
"Your father was never considered smart. he was never considered a good senator. He was only a good vice president because he understood how to kiss Barack Obama's a--," Trump said, referring to his predecessor.
Trump's comments came after the former vice president called for Trump's impeachment. "To preserve our Constitution, our democracy, our basic integrity, he should be impeached," Biden said.
According to Democrats, Trump inappropriately pressured Ukraine to investigate the former vice president and his son's business dealings in the Eastern European country as part of an effort to influence the 2020 presidential election.
Trump has responded by denying any wrongdoing and trying to refocus the conversation on the Bidens. Viktor Shokin, Ukraine's former prosecutor general, has claimed that in 2015, he was told by then-U.S. ambassador to Ukraine Geoffrey Pyatt that an investigation into Burisma Holdings, a natural gas company where Hunter Biden sat on the board, had to be "handled with white gloves, which ... implied do nothing."