[Watch the full video of the debate here.]

Joseph R. Biden Jr. pushed back against President Trump’s attack on his son’s business dealings, saying that the attention should instead be on Mr. Trump’s actions of inviting a foreign power into the election and not on the controversy sparked by unfounded conspiracies about the overseas business dealings of his son Hunter.

But the former vice president at first did not address part of a question at the Democratic presidential debate from Anderson Cooper, who asked whether it was appropriate for his son to have business dealings in Ukraine while Mr. Biden was vice president. Mr. Biden instead pivoted to Mr. Trump, and invoked the founding fathers.

“My son did nothing wrong,” Mr. Biden said. “I did nothing wrong.”

He continued:

“Look, the fact that George Washington on the first time he spoke after being elected, that we had to worry about is foreign interference in our elections, it was the greatest threat to America. This president on three occasions, three occasions, has invited foreign governments and heads of government to get engaged in trying to alter our elections. The fact is that it is outrageous. Rudy Giuliani, the president and his thugs have already proven that they, in fact, are flat lying. What we have to do now is focus on Donald Trump. He doesn’t want me to be the candidate. He is going after me because he knows if I get the nomination, I will beat him like a drum.”

When pressed by Mr. Cooper in a follow-up question on whether his son should have had foreign business dealings during the Obama administration, Mr. Biden again said they had done nothing wrong.