The chairman of the US Senate Judiciary Committee has vowed to investigate allegations that top FBI and justice department officials discussed ways to remove President Trump from office.
Senator Lindsey Graham said the claims were an "attempted bureaucratic coup".
Ex-acting FBI chief Andrew McCabe said Deputy Attorney General Rod Rosenstein had talks in 2017 about a constitutional clause that allows the removal of a president if deemed unfit.
Mr Rosenstein has previously denied it.
The pledge by Mr Graham, a Republican who has become one of the president's biggest defenders, comes after Mr McCabe appeared on US broadcaster CBS saying Mr Rosenstein discussed the numbers needed to invoke the clause, known as the 25th Amendment to the US constitution.
In the 60 Minutes interview aired on Sunday, Mr McCabe also said that:
- The FBI "had reason to investigate" the president's links with Russia, based on Mr Trump's actions
- Mr Rosenstein had been "absolutely serious" when he discussed secretly wearing a wire to record Mr Trump
- Mr Trump said Russian President Vladimir Putin had told him North Korea did not have the capability to hit the US with ballistic missiles - and, when US intelligence officials contradicted this assessment, responded with: "I don't care. I believe Putin"
The White House said Mr McCabe, who was fired last year for allegedly lying to government investigators, had "no credibility".
President Trump has posted several tweets on Monday morning in response to the interview, and what he described as "many lies" by the "now disgraced" Mr McCabe.
He also said that it looked like him and Mr Rosenstein were attempting to carry out a "very illegal act" and a "coup attempt" against his leadership.
“This was an illegal coup attempt on the President of the United States.” Dan Bongino on @foxandfriends True!— Donald J. Trump (@realDonaldTrump) February 18, 2019
What are the 25th Amendment claims?
The allegations that Mr Rosenstein discussed invoking the amendment were first reported last year by the New York Times, which cited anonymous sources.
However, Mr McCabe's quotes are the first to be made on the record from someone present at the meeting where the alleged comments were reportedly made - in May 2017, after Mr Trump fired FBI director James Comey, according to Mr McCabe.
"The discussion of the 25th Amendment was simply [that] Rod raised the issue and discussed it with me in the context of thinking about how many other cabinet officials might support such an effort," he said.
Mr McCabe also said Mr Rosenstein was openly "counting votes, or possible votes" and that he was "very concerned" about the president "his capacity and about his intent at that point in time."
"To be fair, it was an unbelievably stressful time... it was really something that he kind of threw out in a very frenzied chaotic conversation about where we were and what we needed to do next."
Mr Rosenstein has previously strongly denied having such discussions, saying there was "no basis" to invoking the amendment.
Reacting to the interview, Senator Graham described it as "stunning" and pledged to hold a hearing at the Senate Judiciary Committee to determine "who's telling the truth" and that he could issue subpoenas - a court order forcing a witness to appear to give testimony - "if that's what it takes".
The powerful committee he chairs oversees the US judiciary.
"I think everybody in the country needs to know if it happened. I'm going to do everything I can to get to the bottom of Department of Justice [and] FBI behaviour toward President Trump and his campaign," he told CBS.
What is the 25th Amendment?
It provides for the removal of a president if he is deemed unfit for office. Duties are transferred to the vice-president.
Activating the relevant section of the 25th Amendment would require the approval of eight of the 15 members of Mr Trump's cabinet, the vice-president and two-thirds majorities in Congress.
Ronald Reagan and George W Bush used the amendment to temporarily transfer power when they were medically anaesthetised.
What is the claim about secret recordings?
Mr Rosenstein is also alleged to have offered to secretly record Mr Trump, amid concerns about possible obstruction of justice relating to the investigation into alleged collusion between the president's campaign team and Russia.
When the allegations first emerged in the New York Times, Mr Rosenstein said the report was "inaccurate and factually incorrect".
A source told the BBC at the time that Mr Rosenstein's comment "was sarcastic and was never discussed with any intention of recording a conversation with the president".
However, Mr McCabe said that Mr Rosenstein "was not joking. He was absolutely serious".
"It was incredibly turbulent, incredibly stressful. And it was clear to me that that stress was - was impacting the deputy attorney general.
"We talked about why the president had insisted on firing [Mr Comey] and whether or not he was thinking about the Russia investigation. And in the context of that conversation, the deputy attorney general offered to wear a wire into the White House.
"I never actually considered taking him up on the offer," he added.
What did McCabe say about the Russia inquiry?
Mr McCabe said the FBI was right to investigate Mr Trump's ties to Russia.
When Mr Trump had told journalists and Russian diplomats that the Russia inquiry was among the reasons he had fired Mr Comey, his comments indicated that "a crime may have been committed", he said.
Mr McCabe said he was "very concerned" about the Russia case, and wanted to ensure that "were I removed quickly and reassigned or fired, that the case could not be closed or vanish in the night without a trace".
What has the response been?
The justice department says Mr McCabe's account is "inaccurate and factually incorrect".
It also denied Mr Rosenstein authorised any recording or considered invoking the Amendment.
Meanwhile, the White House said: "Andrew McCabe was fired in disgrace from the FBI for lying, and he opened a completely baseless investigation into the president - everyone knows he has no credibility."
Mr McCabe, who took over the FBI in 2017, was himself fired as deputy director in March last year just two days before he was due to retire.
He was sacked by US Attorney General Jeff Sessions, who said an internal review found he leaked information and misled investigators.
Mr McCabe denied the claims and said he was being targeted because of his involvement in the inquiry into alleged Russian meddling in the 2016 election.
He has now written a book on his time in the post.