President Donald Trump's defense team in his impeachment trial turned its attention to Joe and Hunter Biden on Monday, painting the former vice president and his son as corrupt actors who were worthy of being investigated.

"When the House managers gave you their presentation, when they submitted their brief, they repeatedly referenced Hunter Biden and Burisma. They spoke to you for over 21 hours, and they referenced Biden or Burisma over 400 times. And when they gave these presentations, they said there was nothing to see, it was a sham," Pam Bondi, a former Florida attorney general who is among the lawyers on the president's impeachment team, told senators Monday.

Bondi was the first member of the White House defense team to bring up the Bidens, but she was not the last. Another member of the legal team, Eric Herschmann, also spoke at length about the Bidens and Burisma in a subsequent presentation.

Burisma is a Ukrainian gas company that Hunter Biden joined as a board member when his father was vice president.

"In their trial memorandum, the House managers describe this as baseless. Now, why did they say that? Why did they invoke Biden or Burisma over 400 times?" Bondi said.

"The reason they needed to do that is because they're here saying that the president must be impeached and removed from office for raising a concern. And that's why we have to talk about this today," she said. "They say 'sham.' They say 'baseless.' They say this because if it's OK for someone to say, 'Hey, you know what? Maybe there's something here worth raising,' then their case crumbles."

At the heart of the House Democrats' impeachment case is the allegation that Trump abused the power of his office when he tried to condition millions of dollars in military aid to Ukraine on Ukrainian President Volodymyr Zelenskiy's launching investigations of Burisma and a debunked conspiracy theory that Ukraine interfered in the 2016 election.

There has been no evidence of corruption on the part of the former vice president or his son.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

In nearly 30 minutes of remarks Monday, Bondi delved into the minutiae of Hunter Biden's involvement with Burisma, repeatedly saying Democrats must prove beyond a reasonable doubt that there was no basis for Trump to have raised concerns about the Bidens.

She played several video clips, and she displayed a 2014 Washington Post story and read a quotation from it: "The appointment of the vice president's son to a Ukrainian oil board looks nepotistic at best, nefarious at worst."

Her line of defense was a turn away from arguments that Trump's legal team presented Saturday.

Full coverage of President Donald Trump's impeachment trial

One lawyer, Michael Purpura, said Saturday, "There couldn't possibly have been a quid pro quo" — a claim that seems to disregard Bondi's point that Burisma was worthy of investigation.

Bondi closed Monday by claiming, "The House managers might say, without evidence, that everything we just have said has been debunked, that the evidence points entirely and equivocally in the other direction. That is a distraction.

"All we're saying is that there was a basis to talk about this, to raise this issue. And that is enough," she said.

The Biden camp hit back in a caustically worded statement Monday.

"We didn't realize that Breitbart was expanding into Ted Talk knockoffs. Here on Planet Earth, the conspiracy theory that Bondi repeated has been conclusively refuted. The New York Times calls it 'debunked,' The Wall Street Journal calls it 'discredited,' the AP calls it 'incorrect,' and The Washington Post Fact Checker calls it 'a fountain of falsehoods,'" said Andrew Bates, the Biden campaign's rapid response director.

"The diplomat that Trump himself appointed to lead his Ukraine policy has blasted it as 'self serving' and 'not credible,'" Bates said. "Joe Biden was instrumental to a bipartisan and international anti-corruption victory. It's no surprise that such a thing is anathema to President Trump."

Biden himself, informed of Bondi's argument by the members of the news media after a campaign event in Marion, Iowa, said, "Are you kidding me?" before laughing and walking away.

Download the NBC News app for full coverage of the Senate impeachment trial

Herschmann, who presented immediately after Bondi, continued making Bondi's point, questioning how the House managers came to conclude that the desired investigation into the Bidens was a sham.

"The House managers say that the investigations had been debunked, they were sham investigations. So now we have the question: Were they, really?" he said.

"Was it, in fact, true that any investigation had been debunked? The House managers do not identify for you who supposedly conducted any investigations. Who supposedly did the debunking? Who discredited it? Where and when were any such investigations conducted? When were the results published?" he said.

"There's no question that any rational person would like to understand what happened," Herschmann said.