Rudy Giuliani has launched an unhinged attack on John Bolton, calling the former national security adviser a “backstabber” and a “swamp character” for turning on President Trump and publicly accusing him of tying the hold on military aid for Ukraine to an investigation of the Biden family.

Bolton has become Trumpworld’s public enemy No. 1 after it was revealed, in a leaked excerpt of his book first reported by The New York Times, that he alleges the president personally told him that he wanted to continue to withhold nearly $400 million in military aid to Ukraine until officials there pursued investigations into his political rivals.

Giuliani has admitted investigating, on Trump’s direction, unfounded corruption allegations against the Bidens. But, in a CBS News interview broadcast Wednesday morning, Trump’s personal attorney insisted he never once discussed Ukrainian military aid with the president, and said Bolton never confronted him about his controversial work in Ukraine.

“[Bolton] never said to me, ‘I’ve got a problem with what you are doing in Ukraine,’” said Giuliani. “Never once, never winked, never sent me a little note... That’s classic backstabber. So I feel I got a swamp character here. I find his testimony about the president pretty close to incredible.”

The beef between Giuliani and Bolton is long-standing. During congressional testimony in November, former White House aide Fiona Hill claimed Bolton often complained to his colleagues about Giuliani’s work, and his alleged efforts to force out then-U.S. Ambassador to Ukraine Marie Yovanovitch. Hill testified that Bolton compared Giuliani to “a hand grenade that was going to blow everyone up.”

In the CBS News interview, Giuliani criticized Bolton for not bringing up his problems in person, responding: “If he came up to me and said, ‘Rudy, you’re a grenade that’ll blow up,’ and he shows his face, I would say: ‘I would never have the opportunity, ‘cause you’re an atomic bomb.’”

Giuliani went on to say that he first found out about the hold on Ukraine’s aid from media reports, claiming that the president never told him directly that he wanted to withhold aid to win a Biden investigation. “Never ever discussed military aid with the president, with regard to Ukraine,” he said.

Giuliani reportedly added that would be happy to testify in the Senate impeachment trial— if he gets clearance from the White House.