Comey's testimony to the Senate Intelligence Committee starts at 10 a.m. (1400 GMT), in the most widely anticipated U.S. congressional hearing in years.

The outcome could have significant repercussions for Trump's presidency as special counsel Robert Mueller and several congressional committees investigate alleged Russian efforts to influence the 2016 presidential election and whether Trump's campaign colluded with this. Russia has denied such interference and the White House has denied any collusion.

The issue has dogged Trump's first five months in office, with critics saying that any efforts by him to hinder the FBI probe could amount to obstruction of justice. Comey will be making his first public appearance since Trump fired him on May 9, triggering a political firestorm.

The Senate panel released Comey's written testimony on Wednesday, shifting the drama on Thursday to the question and answer period of the hearing.

In his written testimony, Comey quoted Trump as telling him the Russia investigation was a "cloud" impairing his ability to operate as president.

In a one-on-one meeting in the Oval Office on Feb. 14, Comey's statement said, Trump asked him to drop an investigation into former national security adviser Michael Flynn, part of a wider probe into Russian meddling in the election.

"I hope you can see your way clear to letting this go, to letting Flynn go," Comey quoted Trump as saying.

The Senate panel's top Democrat, Senator Mark Warner, will say in his opening statement that Comey's testimony showed Trump violated guidelines put in place after the 1970s Watergate scandal to prevent White House interference in FBI investigations.

"I do want to emphasize what is happening here - the president of the United States is asking the FBI director to drop an ongoing investigation into the president’s former national security adviser," Warner said in excerpts provided to Reuters on Thursday.

'LOYALTY'

After Trump fired Comey, Democrats accused the Republican president of seeking to hinder the Russia probes. Democrats and some Republicans on the committee will use Thursday's hearing to press for further details of any attempts by Trump to blunt the Russia investigation.

Senator Angus King, an independent who votes with Democrats, told CNN he expected many questions would relate to Comey's firing and the conversations that preceded it.

In a detailed account of a series of conversations with Trump, Comey said Trump told him during a one-on-one dinner on Jan. 27 that he needed "loyalty."

Democratic Senator Ron Wyden said on Wednesday he was very concerned about the loyalty comments, adding, "That is another way the president sought to impede the investigation."

Republican Senator Richard Burr, the panel's chairman, sought to play down the remark, saying: "I don't think it's wrong to ask for loyalty from anybody in an administration."

The hearing was expected to be widely watched, with bars in the capital offering "impeachmint" cocktails and $5 Russian vodka shots during the live broadcast.

A line of people snaked through the hallway outside the hearing room waiting to get in, including some Capitol Hill interns who told reporters they were there shortly after 4 a.m.

VINDICATED

Trump's attorney, Marc Kasowitz, released a statement on Wednesday saying the president felt vindicated by Comey's acknowledgement in his written testimony that Trump was not personally under investigation.

Although he has been involved in other political controversies, most notably his handling of the FBI investigation into 2016 Democratic presidential nominee Hillary Clinton's private email server, Comey is widely seen as cautious and fact-oriented.

"One thing you don't ever hear about him is (that) people don't think he tells the truth. He brings a lot of credibility," said Benjamin Wittes, a Comey confidant and senior fellow at the Brookings Institution.

Trump has been impulsive and visceral in the White House, often turning to Twitter to lambaste perceived adversaries. He is widely expected to use Twitter to counterpunch at Comey on Thursday, perhaps even during the hearing.

As Comey's written testimony underscored, he and the president had an awkward relationship.

Then-presidential candidate Trump excoriated Comey last summer for deciding not to prosecute Clinton over her handling of government emails while she was secretary of state. Then he praised Comey when he reopened the issue in October, just days before the election.

After taking office on Jan. 20, Trump initially kept Comey on as FBI director, and publicly embraced him at a January White House event. Two days after firing him, Trump said it was because of "this Russia thing."

During Thursday's hearing, Comey was not expected to directly accuse Trump of trying to obstruct justice by asking him to halt the FBI probe of Flynn. He was also unlikely to reveal new details of the Russia investigation.

U.S. law enforcement officials said Comey had discussed his testimony with Mueller's investigative team to ensure it did not interfere with the special counsel's probe.

"The one thing you know he's not going to do, you know he's not going to reach a conclusion (on the legality of Trump's actions) and he's not going to talk about the underlying investigation," said Stephen Ryan, a former federal prosecutor and congressional investigator now at the McDermott, Will & Emery law firm.

Still, Ryan said the testimony, and senators' questions, would be historic. The closest comparison, he said, was the appearance 44 years ago of President Richard Nixon's White House counsel John Dean, who, after being fired by Nixon, gave damning testimony in 1973 to the Senate Watergate Committee.

(Additional reporting by John Walcott, Doina Chiacu, Susan Heavey)