ANALYSIS: Comey offers central challenge to Trump presidency The former FBI director's testimony "could rock the presidency itself."

 -- Who’s hoping there are no “tapes” now?

With cinematic detail and startling revelations, the testimony former FBI Director James Comey is set to deliver in front of Congress on Thursday offers a central challenge to President Donald Trump -- with ramifications that could rock the presidency itself.

In a president seeking loyalty and hoping for an end to an FBI probe, what Comey lays out may not be anything that was illegal. But it’s also far from a normal depiction of behavior by a president, and will leave the White House and its allies further strained in making the case that Trump’s behavior has been appropriate throughout the crisis that’s defining his time in office.

Comey depicts a president who asked for a pledge of loyalty from the FBI director, just a week after taking office: “I need loyalty, I expect loyalty,” Trump told Comey, according to Comey’s prepared testimony.

Comey also claims that the president directly asked him to drop the investigation of Michael Flynn, Trump’s former national security adviser: “I hope you can see your way clear to letting this go, to letting Flynn go. He’s a good guy. I hope you can let this go.”

Comey’s takeaway, as prepared for Thursday’s hearing: “I had understood the president to be requesting that we drop any investigation of Flynn in connection with false statements about his conversations with the Russian ambassador in December.”

Both points stand in direct contradiction to the official word from the White House. When asked whether a request for Comey’s loyalty ever came from the president, White House press secretary Sean Spicer responded simply: “No.”

And when the president was asked last month if he urged Comey to back down on the Flynn investigation “in any way, shape, or form,” Trump was just as succinct: “No. No. Next question.”

Comey’s account does seem to back up the president in one important respect, at least in part. Trump wrote in his letter firing Comey that Comey told him three times that “I am not under investigation.” Comey documents three times in which he informed Trump that the FBI “did not have an open counter-intelligence case on him.”

"President Trump was right," Republican National Committee Chairwoman Ronna Romney McDaniel said Wednesday, in what figures to be a template for other GOP responses. "Director Comey’s statement reconfirmed what the president has been saying all along -- he was never under investigation."

Without explanation, Comey only details five of the nine one-on-one conversations he recalls having with Trump -- a discrepancy he will surely be asked about Thursday. Comey also confirms that he offered “honest loyalty” to the president -- an unusual phrase Comey said they agreed upon to “end a very awkward conversation.”

With Comey set to have his say, the pushback has already begun. And while there is no White House war room, this is a president at war, with allies already questioning Comey’s veracity and motives.

An outside group with ties to former Trump campaign loyalists is already out with an ad labeling Comey “another D.C. insider” who has been forced to amend and correct previous congressional testimony. That group is promising more anti-Comey messaging in the days to come.

White House adviser Kellyanne Conway went on television this week to remind viewers of Comey’s strained relationships with members both parties.

“Most of Washington, of course many of the Democrats, detested this man until Donald Trump fired him,” Conway said.

The latest ABC News/Washington Post polling suggests an audience that may not be inclined to believe Comey’s word. Fifty-five percent of Americans say they at least somewhat doubt Comey’s word; Trump’s number on that question is 72 percent -- no better, but a reminder of Comey’s partisan tarnishes.

Trump knows his enemies, and their weaknesses. Organized or not, the attacks on Comey will be brutal, with Republican members of the Senate Intelligence Committee set to press Comey on his actions and contradictions.

But Comey doesn’t have a job to hang on to anymore. Trump is the one pressing to establish that he acted appropriately, through an affair that has seemed downright bizarre at times.

Comey describes his final conversation with Trump in memorable terms. He said he got a phone call from the president on April 11, just weeks before he would be fired, asking again for the FBI director to “get out” the fact that the president was not personally under investigation.

Comey said he demurred and told Trump the request should go through Department of Justice channels.

Trump said he would do that, then added: “Because I have been very loyal to you, very loyal; we had that thing you know.” Comey said he did not ask him what “that thing” was.

That scene -- of a president trying to call in a favor based on a personal bit of loyalty -- lingers in the imagination, even if it never becomes clear what “that thing” was. It may be what Trump supporters expect out of a president, but like so many aspects of Trump, there’s no precedent for this.

ABC News’ MaryAlice Parks contributed to this report.