Comey wrote in a memo that Trump asked him to walk away from the Flynn probe, declaring that Flynn was a good man and asserting to his FBI director, "I hope you can let this go."
That, legal analysts say, provides a plausible case that the president obstructed justice. The FBI is investigating Flynn's dealings, as well as possible coordination between the Trump campaign and the Kremlin to influence the 2016 presidential election.
"There's definitely a case to be made for obstruction," said Barak Cohen, a former federal prosecutor who now does white-collar defense work at the Perkins Coie law firm. "But on the other hand you have to realize that — as with any other sort of criminal law — intent is key, and intent here can be difficult to prove."
On Tuesday, all 33 Democrats on the House Judiciary and Oversight and Government Reform committees sent a letter to their Republican counterparts asking to launch an investigation into whether Trump and those in his administration were "engaged in an ongoing conspiracy to obstruct" the various probes by the Justice Department, FBI and Congress. House Oversight Committee Chairman Jason Chaffetz (R-Utah) sent his own letter to the FBI's acting director, asking him to turn over all records of communications between Comey and Trump, noting that reports about the communications "raise questions as to whether the President attempted to influence or impede the FBI's investigation."
The existence of the Comey memo was first reported by the New York Times. Associates of Comey later confirmed the details of it to The Washington Post. Spokesmen for the FBI and the Justice Department declined to comment on it.
The laws governing obstruction of justice require prosecutors to show a person "corruptly" tried to influence a probe — meaning investigators have to find some evidence of what a person was thinking when taking a particular action.
In this case, analysts said, that would mean analyzing the specific details of Trump and Comey's conversation, assessing what else was happening at the time and possibly talking to Trump associates who had talked with the president about what he wanted to do.
"It depends on what he said and how he said it," said Edward B. MacMahon Jr., another criminal defense lawyer. "I call all the time and ask prosecutors to stop investigations. It just depends on how it's done."
MacMahon noted there are circumstances — such as the CIA worrying about disclosure of classified information — in which one component of the executive branch discourages prosecutors from pursuing a case.
"If the reason you wanted it stopped was because it was going to lead to the prosecution of one of your friends, that would be one thing," he said. "If you wanted it stopped because you didn't want to disclose national security information, that's something provided by statute."
In a statement, the White House disputed Comey's characterization of his conversation with Trump, saying "the president has never asked Mr. Comey or anyone else to end an investigation, including any investigation involving General Flynn. . . . This is not a truthful or accurate portrayal of the conversation between the president and Mr. Comey.''
That is important, analysts said, because if the president were merely asserting that "he personally knows the target of the investigation and he's just trying to provide his two cents," that might not present him any legal problems, Cohen said.
"You have Comey's word against Trump, right?" Cohen said. "You need somebody else that Trump may have spoken to that provides evidence of intent."
Trump has suggested on Twitter that there might exist tapes of his conversations with Comey. If that is true, those could be critical to proving whose account of the meeting is accurate.
At worst, Cohen said, prosecutors' theory of the case would be that Trump pressured Comey to stand down on Flynn because he feared the investigation could affect him personally. Last week, Trump removed Comey from his post as FBI director and claimed that Comey had told him three times he was not under investigation.
"While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau," Trump wrote to Comey.
Cohen said that itself could be a piece of circumstantial evidence that Trump was trying to impede an ongoing probe. Trump also said in an NBC News interview that the Russia probe was on his mind when he fired his FBI director.
Cohen said Comey's memo was "direct evidence" against Trump, but he noted "it only comes out after the termination."
"The question is, if the president committed the crime or attempted to commit the crime of obstruction, and you serve the Constitution and not the president, why didn't you say something about this before you were fired?" Cohen said.
When the FBI was investigating Hillary Clinton's use of a private email server while she was secretary of state, President Barack Obama notably said in an interview with Fox News, "I continue to believe that she has not jeopardized America's national security." Obama insisted the White House was not meddling in the probe — though his comments undeniably struck at the heart of the matter.
Cohen said that while it was "highly improper" for Trump to insert himself in an investigation in any way, charging him would be difficult. With a lower-profile target, he said, "maybe prosecutors might be aggressive enough to bring a case," but "it also arguably undermines democracy for prosecutors to go after a sitting president with only circumstantial evidence."
The Justice Department's Office of Legal Counsel has opined in the past that the president cannot be indicted or prosecuted at all, "because it would impermissibly interfere with the President's ability to carry out his constitutionally assigned functions and thus would be inconsistent with the constitutional structure."
Trump could be impeached, though, for "treason, bribery, or other high crimes and misdemeanors." In the articles of impeachment against Richard Nixon — which were never taken up by the full House of Representatives — legislators cited obstruction of justice.