Rudy Giuliani, the Trump attorney heavily involved in the Ukraine affair, called former National Security Adviser John Bolton a “backstabber” over his comments about U.S. aid to Ukraine being conditioned on the foreign government agreeing to launch a political probe.

“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.”

Giuliani said he doubted Bolton’s claim that President Donald Trump explicitly told him he wanted to freeze the aid until Ukraine announced an investigation into Trump’s political rival, Joe Biden.

“I find his testimony about the president pretty close to incredible,” Giuliani said. “I can’t imagine that the president of the United States said that to him.”

According to The New York Times, Bolton wrote in an unpublished manuscript that Trump told him he did not want to release about $400 million in military aid to Ukraine unless the country agreed to open investigations into the Biden family.

The manuscript was sent to the White House in late December to be reviewed for any classified information, before Bolton publishes his book.

“It is clear, regrettably, from the New York Times article published today that the pre-publication review process has been corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript,” Bolton’s lawyer, Charles Cooper said Sunday night.

Bolton’s accusation is the first to directly link Trump to a quid pro quo deal with Ukraine. Since the New York Times report, several Democrats have called for his testimony in the Senate impeachment trial.

A majority vote would be required to call any witnesses, requiring at least four Republican senators to flip.

Bolton has previously indicated he would be willing to testify.

White House counsel Alan Dershowitz said Monday that even if Bolton’s claims are true, it still does not constitute an impeachable offense.

“Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense,” Dershowitz said. “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.'”