(Reuters) - (The opinions expressed here are those of the author, a columnist for Reuters.)

In his letter firing FBI Director James Comey on Tuesday, President Trump thanked Comey for informing him on three occasions that Trump was not under investigation.

If Comey offered the president such assurances, in the early days of a sensitive FBI probe of possible ties between Trump campaign officials and the Russian government, his disclosures were inappropriate under Justice Department protocols.

But if the president’s account is false, Trump tarnished Comey’s reputation as he pushed the FBI director out the door – and gave him a powerful incentive to defend himself.

On Wednesday, the Senate Intelligence Committee, headed by chairman Richard Burr, a North Carolina Republican, and vice-chair Mark Warner, a Virginia Democrat, invited Comey to tell his side of the story on Tuesday in a closed-door interview. Comey is not expected to provide formal testimony, but it’s going to be very interesting to see how the former FBI director and the White House respond to the invitation, especially because the president’s own words may end up restricting his options.

The Senate, according to Savannah Law professor Andy Wright, a former associate counsel in the Obama White House, is going to want to know the basis of the president’s assertion that Comey assured him he was not being investigated in the FBI’s Russia probe. Wright and Alberto Gonzales, White House counsel and attorney general for George W. Bush, both told me it would have been inappropriate for Comey to have offered Trump that assurance.

“I don’t think it would have been appropriate for Comey to have commented on the investigation in any way to the president,” said Gonzales. The Justice Department, which includes the FBI, has strict procedures limiting what prosecutors and investigators can disclose to White House officials to avert any taint of presidential interference.

But if Comey’s recollection of his disclosures does not match the president’s version of their conversations, the former FBI director “is entitled to defend his conduct,” Gonzales said. In fact, he said, Comey will likely be eager for the chance to tell his story. “If the conversations did not occur and I’m being fired and the letter firing me puts me in an additional bad light, I’d be pretty angry,” said Gonzales.

Comey, who has a long and distinguished career as a federal prosecutor, is an old pro at appearing before Congress. In 2007, he gripped the Senate Judiciary Committee with an account of his 2004 race to the hospital bedside of Attorney General John Ashcroft. Comey, who was then Deputy AG, said he arrived just in time to stop then White House Counsel Gonzales from pressuring the gravely ill Ashcroft to reauthorize a domestic surveillance program the Justice Department had deemed illegal.

There are no ethical strictures to stop Comey from telling Congress about his conversations with the president, according to law professor Wright and Eric Columbus, a former senior counsel to the Deputy AG in the Obama administration. Comey cannot reveal classified information or investigative details, even as a private citizen, but Wright and Columbus said Comey’s conversations with President Trump are not shielded by attorney-client privilege because the president was not Comey’s client. (As FBI head, Comey was not even acting as a lawyer, although, of course, he is one.)

The more intriguing question is whether the White House will assert executive privilege to try to stop Comey from talking to the Senate committee. One component of executive privilege shields presidential communications. But Wright and another law professor, Kathleen Clark of Washington University, said the privilege over presidential communications can be overcome by a compelling government interest in disclosure.

The precedent establishing the bounds of privilege for presidential communications came in the U.S. Supreme Court’s 1974 decision in U.S. v. Nixon, in which the justice ordered a Watergate-besieged Richard Nixon to turn over audiotapes and documents to special prosecutor Leon Jaworski.

So if the White House does attempt to invoke executive privilege to silence Comey, Wright said, Watergate will inevitably be a big part of the debate. That’s a potentially high political price for what law profs Wright and Clark said would probably be a losing legal argument for the Trump administration. One of the critical vulnerabilities for the White House, according to the professors, is Trump’s own public description of his conversations with Comey. Those could be considered a waiver of any presidential privilege protecting Trump’s interactions with the former FBI director.

The White House, as Wright recounted at the Just Security blog, previously backed down from an assertion of presidential communications privilege over scheduled testimony by fired Acting Attorney General Sally Yates, although it’s not clear whether the administration failed to respond to a challenge from Yates’ lawyer on the merits or because the House Intelligence Committee canceled the March hearing at which Yates was scheduled to testify. Yates later appeared before same Senate committee that has invited Comey for an interview next week.

“If the president asserts executive privilege, that’s an aggravating factor in this political climate,” Wright said. “No doubt about it, Comey is a wild card with serious downside risk for the president.”