A key ally of Donald Trump has decried alleged efforts by senior officials to depose the president as a “bureaucratic coup” and pledged to hold a congressional investigation.

Republican senator Lindsey Graham said it was “stunning” the former acting director of the FBI, Andrew McCabe, had admitted in a TV interview he had discussed invoking the 25th Amendment – which allows the cabinet to remove a president from office – with the deputy attorney general Rod Rosenstein.

“Was this an attempted bureaucratic coup? I don’t know. I don’t know who’s telling the truth,” he told CBS News.

“I know Rosenstein vehemently denied it but we’re going to get to the bottom of it.

“I promise your viewers the following; that we will have a hearing about who’s telling the truth, what actually happened.”

Will Trump fall to the 25th Amendment? The key players in the debate Show all 7 1 /7 Will Trump fall to the 25th Amendment? The key players in the debate Will Trump fall to the 25th Amendment? The key players in the debate President Trump Donald Trump, whose firing of the FBI director James Comey in 2017 sparked concerns among senior government officials Getty Will Trump fall to the 25th Amendment? The key players in the debate James Comey The former director of the FBI, James Comey, whose firing by Mr Trump sparked the crisis in 2017 EPA Will Trump fall to the 25th Amendment? The key players in the debate James McCabe James McCabe, the former acting director of the FBI, who has made the explosive accusations in a TV interview AFP Will Trump fall to the 25th Amendment? The key players in the debate Rod Rosenstein The deputy attorney general Rod Rosenstein, who allegedly discussed which cabinet members might back invoking the 25th Amendment Reuters Will Trump fall to the 25th Amendment? The key players in the debate Lindsey Graham Republican senator Lindsey Graham, a key Trump ally who has pledged to hold an investigation into the affair AFP Will Trump fall to the 25th Amendment? The key players in the debate Christopher Wray The new director of the FBI, Christopher Wray, who took over after Mr Trump fired Mr Comey AP Will Trump fall to the 25th Amendment? The key players in the debate Mike Pence The vice-president Mike Pence, who has dismissed suggestions Mr Trump is not capable of discharging his duties AFP

Mr Graham is the chairman of the Senate judiciary committee.

Mr McCabe said last week Mr Trump’s sacking of FBI boss James Comey had so disconcerted Justice Department officials they openly spoke about whether he might be deposed through the 25th Amendment.

Mr Rosenstein has disputed allegations he discussed removing Mr Trump and connected claims he proposed wearing a wire to secretly record Oval Office conversations.

But on Sunday it emerged at least two senior FBI lawyers had testified in private to congressional committees about the 25th Amendment episode, corroborating Mr McCabe’s account.

Mr Graham, a one-time critic of Mr Trump who is now one of the president’s loudest defenders, told CBS News any effort to use the constitutional mechanisms in the 25th Amendment would amount to an “administrative coup”.

“I’m going to do everything I can to get to the bottom of the Department of Justice FBI behaviour toward President Trump and his campaign.

“If it happened we need to clean it up. The FBI has gotten off track in the past. There is no organisation that can’t withstand scrutiny.”

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When asked if he would even issue legal summons to force Mr McCabe and Mr Rosenstein to give evidence in Congress, Mr Graham said: “How can I not, if that’s what it takes?

“We’re going to find out what happened here and the only way I know to find out is to call the people in under oath and find out, through questioning, who’s telling the truth because the underlying accusation is beyond stunning.”

But a Democratic member of Mr Graham’s committee, Chris Coons, said his colleague was overreacting.

“I don’t think that this frankly rises to the level of some deep state conspiracy or a serious attempt at what Senator Graham called an administrative coup.

“I suspect that once this is fully discussed, it’ll be clearer that this was a brief or passing conversation that’s been taken out of context.”

Shortly after Mr McCabe’s TV interview aired, Mr Trump replied via an angry series of tweets.

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“Disgraced FBI Acting Director Andrew McCabe pretends to be a ‘poor little Angel’ when in fact he was a big part of the Crooked Hillary Scandal & the Russia Hoax – a puppet for Leakin’ James Comey,” he said.