It's being called the Super Bowl of Washington politics.

Fired FBI director James Comey's upcoming testimony to the Senate Intelligence Committee is the talk of the town.

Bars are opening early at 9:30am (local time) and switching their TVs to one of the major cable news networks that will provide live coverage of the hearing.

Local cable networks even have a live counter until Mr Comey's hotly anticipated testimony. ( Supplied: Youtube )

"I don't think we've seen a congressional hearing of this kind since Watergate," Reuters national security correspondent Jonathan Landay told the ABC.

It's the first time Mr Comey will speak publicly since he was abruptly sacked by US President Donald Trump while the head of a probe into possible links between Trump associates and Russia.

His testimony could have significant ramifications.

What will Comey say?

The Senate Intelligence Committee has released Mr Comey's seven-page opening statement ahead of the hearing.

In it Mr Comey refers to detailed memos he says he wrote after each one-on-one conversation with the President.

He says Mr Trump asked him about the investigation into sacked National Security Adviser Michael Flynn and said "I hope you can let this go".

Mr Comey writes that he "understood the President to be requesting that we drop any investigation of Flynn" and that he found the conversation "very concerning".

He also says Mr Trump pressed him on loyalty, and twice pressured him to publicly state that the President was not personally being investigated for links to Russia.

Mr Comey says he informed other senior FBI officials about the meetings and asked Attorney General Jeff Sessions to "prevent any future direct communication" between him and the President.

Obstruction of justice

Committee members may press Mr Comey on whether he thinks Mr Trump was trying to obstruct the course of justice by asking him to drop the investigation into Mr Flynn.

Obstruction of justice was one of the articles of impeachment levelled against presidents Richard Nixon and Bill Clinton.

While the release of Mr Comey's opening statement has prompted some Democrats to call for impeachment, that decision would be made by Congress which is currently controlled by Republicans.

"There is a general agreement that impeachment is an extraordinary constitutional remedy, and early in a matter like this," Bob Bauer, White House counsel under the Obama administration, told ABC News.

"Only months after a general election it is not simply part of the American political culture to start contemplating the ouster of a newly elected President," he said.

'He said, he said'

Mr Trump has denied he asked Mr Comey to drop the investigation into Mr Flynn.

He's also called Mr Comey a "showboat" and reportedly a "nutjob". In a veiled threat, Mr Trump also suggested there might be "tapes" of the pairs' conversations.

"I think Mr Comey is probably there to defend his reputation and to defend his veracity. Here he is being insulted essentially by the President of the United States, his integrity called into question and here is Mr Comey's opportunity to fight back," Mr Landay said.

The hearing could lead to a showdown between the most powerful man in America and a widely respected career bureaucrat.

"You set up a kind of 'he said, he said' situation so that it comes down to who has — who commands — the most trust in terms of the American public the President of the United States or his former FBI director," Mr Landay said.

How Trump supporters and Republicans respond

Trump supporter Joy Villa believes the Russia probe is a witch hunt. ( ABC News: Stephanie March )

Mr Trump's approval rating has slipped in recent weeks as allegations surrounding the Russia probe emerged. A recent Washington Post poll shows a majority of Americans have concerns about his firing of Mr Comey.

Republican members of Congress who are up for re-election in 2018 will be paying close attention to the polls as they decide whether it is in their political interest to stand by Mr Trump. His approval rating among Republicans is down to 75 per cent.

Many of his supporters continue to see the Russia probe as witch hunt and dismiss the leaks and allegations as fake news.

"This Russia story is getting so old," Trump supporter Joy Villa said at a prayer meeting outside the White House.

"I believe there is a lot of making up smoke so that people assume there is fire. I think there is a lot of thinking that the American public is stupid and if you just say 'Russia' they are going to say 'oh, it's scary'.

"I don't believe it is true".

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