There has been no evidence of wrongdoing by either Joe or Hunter Biden. The former vice president also denounced Trump, his personal lawyer Rudy Giuliani and their "thugs" as liars.
Other candidates onstage also seemed to want to move on from Ukraine, despite Biden's position as one of the campaign's front-runners.
Hunter Biden said he did not discuss his foreign business dealings with his father.
He joined the board of a gas company in Ukraine more than half a decade ago when his father was the Vice President during the Obama government, trying to control corruption in Ukraine.
Trump had ordered the withholding of US military aide to Ukraine unless President Volodymyr Zelensky agreed to investigate Biden and his son Hunter, who had served on the board of a Ukrainian natural gas company.
In 2014, then-Vice President Joe Biden was at the forefront of American diplomatic efforts to support Ukraine's fragile democratic government as it sought to fend off Russian aggression and root out corruption.
"The only person at home enjoying that was Donald Trump", Booker said.
"He goes into a meeting and walks out with $1.5 billion", the president said, adding, "these are the same Chinese negotiators I'm dealing with". "That is what we should be focusing on".
The presenter was supposed to report on the rally, but said he would not allow the audience to hear the president's words about Joe Biden and his son Hunter because, according to him, the president had "crossed a line".
The contradiction prompted a follow-up from moderator Anderson Cooper, who noted that Hunter Biden had "admitted that he made a mistake and showed poor judgement by serving on that board in Ukraine".
"I never discussed a single thing with my son about anything having to do with Ukraine", Biden maintained, including when Hunter was the attorney general of Delaware.
Later he said, "My son made a judgment. No one has indicated I have".
"I made a mistake, in retrospect, as it related to creating any perception that it was wrong, and so, therefore, I'm taking it off the table", he said in the interview. The fact of the matter is this is about Trump's corruption. "We'll see if anybody else makes that commitment, but that's the commitment I'm making". 'He answered all the questions, ' she said.