By Warren Strobel and Patricia Zengerle

WASHINGTON (Reuters) – Former FBI Director James Comey will tell Congress on Thursday that President Donald Trump pressed him repeatedly to halt a probe into his ex-national security adviser’s ties with Russia and to declare publicly that Trump himself was not under investigation.

Comey’s testimony in the most widely anticipated congressional hearing in years will put at center stage a high-stakes clash between two men with vastly different personas.

The outcome could have significant repercussions for Trump’s 139-day-old presidency as special counsel Robert Mueller and multiple congressional committees investigate whether Trump’s campaign team colluded with Russia during the 2016 presidential election. The White House and Russia deny any collusion occurred.

In written testimony released by the Senate Intelligence Committee on Wednesday, Comey quoted Trump as telling him the Russia investigation was a “cloud” impairing his ability to operate as president.

Comey said in his statement that in a one-on-one meeting in the Oval Office on Feb.14, Trump asked him to drop an investigation of former national security adviser Michael Flynn that is part of a wider probe into Russia’s alleged meddling in the 2016 presidential election.

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

Comey also said Trump told him during a one-on-one dinner on Jan. 27 that he needed “loyalty.”

Trump fired the FBI chief on May 9, setting off a political firestorm, and he has since called Comey a “showboat” and a “grandstander.”

Democrats, along with some Republicans, on the committee will use the hearing on Thursday to press for further details of any attempts by Trump to blunt the Russia investigation.

“I’m very concerned about the implication that Comey keeping his job was dependent on his loyalty or, in Comey’s words, developing a ‘patronage relationship.’ That is another way the President sought to impede the investigation,” Democratic Senator Ron Wyden said in a comment emailed to Reuters.

Senator Susan Collins, a Republican member of the panel, said earlier this week: “I want to know more also about the president’s interactions with Mr. Comey with regard to the investigation into Michael Flynn. … It makes a big difference what the exact words were, the tone of the president, the context of the conversation.”

But Republican Senator Richard Burr, the panel’s chairman, sought to downplay Comey’s “loyalty” remark, saying: “I don’t think it’s wrong to ask for loyalty from anybody in an administration.”

Trump’s attorney, Marc Kasowitz, released a statement on Wednesday saying the president felt “totally vindicated” by Comey’s acknowledgement that he had told Trump on three occasions that he was not personally under investigation.

Despite landing himself in other political controversies, including his handling of the FBI investigation of 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.

Less than five months into office, Trump has proven himself to be impulsive and visceral, turning to Twitter to lambaste perceived adversaries in 140 characters or less.

AWKWARD RELATIONSHIP

As Comey’s written testimony underscored, he and the U.S. president had an awkward, topsy-turvy relationship.

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

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.”

Trump is widely expected to use his Twitter account, which lists 31.8 million followers, to counterpunch at Comey on Thursday – perhaps even in real time.

The Republican president’s unconstrained use of Twitter has confounded allies and skeptics alike.

“Every time you tweet, it makes it harder on all of us who are trying to help you. I don’t think you did anything wrong. Don’t get in the way of an investigation that could actually clear you,” Republican Senator Lindsey Graham told Fox News on Wednesday.

Despite the high drama, Comey is not expected to drop any major new bombshells, or directly accuse Trump of trying to obstruct justice by asking him to halt the FBI probe of Flynn.

He is also unlikely to reveal new details of the ongoing 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; Editing by Yara Bayoumy and Peter Cooney)