James Comey: World is watching as ex-FBI boss faces Senate
Whether Mr Trump's conduct amounts to obstruction of justice depends on how much the sacked FBI director is willing to say.
Thursday 8 June 2017 15:53, UK
James Comey's appearance before the Senate Intelligence Committee is among the most dramatic congressional testimonies in America's history.
Mr Comey, who was fired as FBI director last month, could cause real problems for Donald Trump.
For a few hours, the nation will be watching on. In Washington DC, bars are gearing up for a blockbuster moment.
Shaw's Tavern is serving up a shot of Russian vodka with people's morning "covfefe".
It's hosting "Comey hearing Covfefe", with FBI sandwiches - fried chicken, bacon and iceberg lettuce - available as people watch events unfold on their screens.
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Sky News is broadcasting the coverage, which is set to begin at 10am (3pm UK time).
Mr Comey has said it is the only public testimony he will give on recent events.
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During Tuesday's White House press briefing, press secretary Sean Spicer wouldn't be drawn on whether Mr Trump will be watching, simply telling reporters he "has a full day on Thursday".
Mr Trump, meanwhile, said "I wish him luck" when asked about the man he fired.
He is expected to give a speech towards the end of Mr Comey's hearing. Many in Washington are speculating about the prospect that the social media-driven leader might live tweet reaction.
A key question hanging over the hearing is whether Mr Comey will outline whether Mr Trump's conduct amounts to obstruction of justice.
On Tuesday, ABC News reported that he wouldn't make that allegation at the stand.
Their source said: "He's there to share his concerns."
Two encounters between Mr Comey and Mr Trump have been at the centre of those concerns.
At a dinner on 27 January, the President allegedly pressed Mr Comey at least twice to pledge his "loyalty" to him.
Mr Comey allegedly only promised to be honest. The White House rejects this account.
Mr Comey, an avid note taker, is believed to have kept a memo detailing a later meeting with Mr Trump on Valentine's Day, the day after Michael Flynn had been fired for giving misleading accounts of his conversations with Russian ambassador Sergey Kislyak.
It's claimed the memo asked Mr Trump to drop the FBI investigation into Mr Flynn's links with Russia.
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The President allegedly told Mr Comey "I hope you can let this go" and described Mr Flynn as a "good guy".
Mr Trump denies leaning on him in any way.
The President claims Mr Comey told him on three occasions that he was not the subject of an FBI investigation.
He is expected to heavily push back against that suggestion.
The impact of the hearing could be negligible, or it could be a catalyst for steps towards impeachment.
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However, Mr Comey, who has been criticised for his interventions, is expected to walk a careful line.
He has met the special prosecutor in the Russian probe, Robert Mueller, ahead of the hearing and has reportedly discussed clear parameters for what he can and cannot discuss in the course of an active investigation.
Democrat Al Green was the first congressman to call for Mr Trump's impeachment, claiming the President has obstructed justice.
He told me he plans to draft articles of impeachment against President Trump despite what he says have been multiple death threats.
"The facts are simple and indisputable," he claimed.
Ron Hosko, former FBI assistant director, told me he believes calls for impeachment are "premature".
However, he went on to say: "If Mueller and Comey think he may be witness (to obstruction of justice) then I think he's going to have pre-scripted very closely defined comments on what those conversations look like and contained if his testimony is needed later."