The Justice Department official whose recommendation has been cast as the main reason why President Donald Trump decided to fire FBI director James Comey, reportedly threatened to resign.

According to one report, Mr Trump had long questioned Mr Comey’s loyalty and judgment, and was unhappy by what he viewed as the director’s lack of action on leaks from within the federal government. Mr Trump officially decided to dismiss Mr Comey based on comments from Deputy Attorney General Rod Rosenstein, according to documents released by the White House.

In a memo to Attorney General Jeff Sessions, Mr Rosenstein said he could not defend Mr Comey’s “refusal to accept the nearly universal judgment that he was mistaken” in the way he handled an investigation into Hillary Clinton’s use of a private email server while she was Secretary of State.

But the Washington Post said Mr Rosenstein was deeply upset about taking the blame for the president's decision and threatened to quit. ABC News also reported that Mr Rosenstein was on the verge of resigning, upset that the White House had pinned him as the reason for the Comey firing.

On Capitol Hill, Vice President Mike Pence said that when Mr Rosenstein “brought the recommendation to the president that the director of the FBI should be removed, President Trump provided the kind of strong and decisive leadership the American people have come to be accustomed from him”.

Deputy press secretary Sarah Huckabee Sanders said during a White House press briefing that Mr Trump had been contemplating firing Mr Comey for awhile.

Mr Comey, an Obama administration appointee, was also leading a probe into alleged Russian meddling in the 2016 US election and alleged links between Moscow and the Trump campaign.

Democrats, as well as few Republicans, have criticised the abrupt firing, saying they are troubled by the reasoning and timing of Mr Comey’s termination.

On the Senate floor, Majority Leader Mitch McConnell noted that Democrats had repeatedly condemned Mr Comey in the past, even calling for his removal. He pointed out that current Senate Democratic leader Charles Schumer said last year that Mr Comey’s handling of the email investigation “appeared to be an appalling act, one that he said goes against their tradition of prosecutors at every level of government.”

“It’s also clear what my Democratic colleagues think of the man who evaluated Mr Comey’s professional conduct and concluded that the bureau needed a change in leadership,” Mr Connell said. “The Democratic leader just a few weeks ago praised Mr Rosenstein for his independence and said he had developed a reputation for integrity.”

The Senate voted 94-6 to confirm Mr Rosenstein as deputy attorney general in April.