President Trump's personal attorney Rudy Giuliani on Tuesday called former national security adviser John Bolton a "backstabber" in response to explosive claims about Mr. Trump and the pause in aid to Ukraine. Giuliani also expressed disbelief that Mr. Trump had ever told Bolton the release of military aid to Ukraine was contingent upon Ukraine investigating the president's political rivals.

"He never said to me, 'I've got a problem with what you are doing in Ukraine,'" Giuliani said in an interview with CBS News. "Never once, never winked, never sent me a little note. He is a personal friend, I thought. So here's the only conclusion I can come to, and it's a harsh one, and I feel very bad about it: He's a backstabber."

When confronted with the seriousness of leveling such a claim against a former administration official, Giuliani doubled down.

"It's not serious, it's true. If your friend was complaining about you behind your back and didn't have the guts to come up to your face and tell you, 'I think you're screwing up, Catherine,' that'd be a backstabber. That's classic backstabber. So I feel I got a swamp character here."

"I find his testimony about the president pretty close to incredible," Giuliani added. "I can't imagine that the president of the United States said that to him."

According to a report from The New York Times, Bolton wrote in an upcoming book that Mr. Trump had told him he did not want to release nearly $400 million in military aid to Ukraine unless the country agreed to open investigations into Democrats and Joe and Hunter Biden. A manuscript of the book was sent to the White House for review in December, according to the report.

If the report is accurate, Bolton would be the first person to attest that Mr. Trump himself expressed the desire for this quid pro quo arrangement. This directly contradicts the president's defense of his actions with Ukraine.

He has stated repeatedly there was "no quid pro quo." The allegation that Mr. Trump withheld the funding in order to get Ukraine to investigate the Bidens lies at the center of his impeachment.

CBS News has requested comment from the White House legal team, as well as an attorney for Bolton.

Democrats have seized on the report, demanding that Bolton testify in the Senate impeachment trial. Several Republican Senators, including Susan Collins and Mitt Romney, have expressed an openness to voting to allow witness testimony in the trial. Several attempts to guarantee that witnesses would testify in the impeachment trial failed early in the Senate proceedings.

White House counsel Alan Dershowitz on Monday addressed Bolton's claims during trial arguments, saying that even if they are true, Mr. Trump's actions do not rise to the level of an impeachable offense.

"It follows from this that if a president, any president, were to have done what the Times reported about the content of the Bolton manuscript, that would not constitute an impeachable offense," Dershowitz said.

He added: "Let me repeat. Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense. That is clear from the history, that is clear from the language of the Constitution. You cannot turn conduct that is not impeachable into impeachable conduct simply by using words like 'quid pro quo' and 'personal benefit.'"

Giuliani's full interview will air on "CBS This Morning" on Wednesday.

Jordan Freiman contributed to this report.