Rudy Giuliani has lashed out at former national security adviser John Bolton, accusing him of being a “backstabber” and a “swamp character.”

Mr Bolton earned the ire of those loyal to Donald Trump when The New York Times reported that his upcoming book contains passages suggesting Mr Trump personally shared his plan to withhold military aid to Ukraine until they agreed to investigate his political opponents.

Giuliani has admitted to investigating allegations of corruption charges against the family of Joe Biden, but denies ever discussing the issue of Ukrainian military aid Mr Trump.

Discussing Mr Bolton during a CBS News interview, Mr Giuliani said the former national security adviser never shared any apprehension with him regarding the investigation into the Bidens.

“Never once. Never winked, never sent me a little note. That’s classic backstabber,” Mr Giuliani told CBS News. “I feel like I’ve got a swamp character here.”

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

Though Mr Giuliani has referred to Mr Bolton as a friend, the men have frequently been at odds.

During her testimony to the US House impeachment investigators, National Security Council senior director Fiona Hill – who reported to Mr Bolton – testified that the then-national security advisor was troubled by Mr Giuliani’s actions in Ukraine, referring to the president’s personal lawyer as a “hand grenade that would blow everyone up.” As a result, Mr Bolton asked Hill to keep notes on the Ukraine situation and share them with the National Security Council’s chief lawyer.

Mr Giuliani complained that Mr Bolton didn’t make the comments to him personally, and went on to say if he was a “hand grenade” then Mr Bolton was an “atomic bomb.”

“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,’” Mr Giuliani said.

The former New York mayor is echoing the same message Mr Trump shared on Twitter on Monday.

Then, on Wednesday, Mr Trump used similar language to Mr Giuliani to refer to Mr Bolton as a destructive force.

Senate Democrats want to use Mr Bolton as a witness in the impeachment trial of Mr Trump, and the former national security adviser said he would testify if subpoenaed by the Senate.

Senate Majority Leader Mitch McConnell hoped to block the Democrats from calling witnesses, though it is unclear if Mr McConnell has enough votes to stop them. Senator Lindsey Graham, a consistent Trump loyalist, pleaded with Bolton to “hold a news conference and we’ll consider what you’ve got to say if you think it’s that important.”

Mr Giuliani isn’t even affording Mr Bolton’s testimony that much consideration.