At the moment, on May 9th, when President Trump abruptly fired the F.B.I. director James Comey, Trump was not under investigation. He very well may be under investigation now.

That was the key revelation in three astonishing hours of American political history, in which Comey, a calm, confident, aggrieved witness before the Senate Intelligence Committee, accused the White House of “lies, plain and simple,” faulted Trump for trying to conceal an improper request, and left no doubt that, in Comey’s view, the facts will eventually demonstrate that the President tried to obstruct a criminal investigation of Trump’s close associate Michael Flynn. It was worth pausing to distinguish how truly rare this instance is: this was not a political partisan tossing off a criticism of a rival; this was a career prosecutor, who served Republican and Democratic Presidents, presenting a time line of specific statements from the President that he described as either untrue or potentially criminal.

From his opening words, Comey made clear that, as a private citizen, he is no longer confined to the narrow, self-edited comments that defined his years at the Justice Department and the F.B.I. He delivered an opening statement that offered a full-throated defense of the thirty-five thousand men and women at the F.B.I., in effect calling on them, from beyond the professional grave, to carry on their work without fear of White House intimidation. Comey also bluntly framed the Trump White House as a duplicitous agent of political obfuscation. Of his firing, Comey said, “The Administration chose to defame me and, more importantly, the F.B.I. by saying the organization was in disarray, that it was being poorly led, that the workforce had lost faith in its leader. Those were lies, plain and simple.”

It will take days to flesh out each of Comey’s feast of offerings, including his claim that the former Attorney General Loretta Lynch told him to call the Hillary Clinton e-mail probe a “matter,” which the campaign preferred, rather than an “investigation”; that Trump’s tweet about the possibility of White House “tapes” inspired Comey to leak memos of his conversations with the President, in order to protect himself (“Lordy, I hope there are tapes,” he said); and that the sitting Attorney General, Jeff Sessions, may be under scrutiny for Russian contacts beyond those that he has acknowledged. In total, Comey’s testimony left a disturbing portrait of the ways that federal law enforcement is threatened, if not always successfully, by the risk of political influence and obfuscation.

But what will matter most in the months of grinding investigation to come is what Trump said and did in his efforts to thwart the investigation of Flynn, his campaign loyalist, who was fired for lying about contacts with the Russian Ambassador. Less than ten minutes into the hearing, the central issue became clear. The committee’s chairman, Richard Burr, a Republican from North Carolina, asked Comey about his conversation with Trump in the Oval Office on February 14th. According to Comey’s notes of the meeting, the President said, “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.” Burr asked Comey, a former prosecutor, if he thought Trump had sought to obstruct his work.

“I don’t think it’s for me to say whether the conversation I had with the President was an effort to obstruct,” he replied, adding, “I took it as a very disturbing thing, very concerning. But that’s a conclusion I'm sure the special counsel will work towards to try and understand what the intention was there and whether that’s an offense.”

That Comey is “sure” the special counsel, Robert Mueller, will try to answer the question bespeaks a range of potential criminal outcomes. Obstruction of justice is a federal crime, though no President has ever been charged with it. In the cases of Richard Nixon and Bill Clinton, the Justice Department ultimately decided not to pursue charges, but the process of investigating the accusations produced underlying facts with vast consequences: they gave rise to congressional proceedings that laid the grounds for impeachment.

There are other potential courses for prosecutors to take: after Comey discussed his belief that Trump was trying to draw him into a “patronage” relationship, Charlie Savage, who covers the law for the Times, wrote, “A former federal prosecutor texted me that the patronage quid pro quo stuff could be a separate crime: trading an official act (keeping public job) for private gain—like the former Illinois governor Rod Blagojevich and Obama’s vacated Senate seat.”

Over and over, Comey mentioned the importance, in his mind, of the fact that Trump had asked other senior officials to leave the Oval Office before he made the request. Asked why the public should believe him, Comey held his credibility up to the President’s and, in effect, asked Americans to judge them. “I think people should look at the whole body of my testimony. . . . Take it all together, and I’ve tried to be open and fair and transparent and accurate. A really significant fact to me is, Why did he kick everybody out of the Oval Office? Why would you kick the Attorney General, the President, the chief of staff out to talk to me if it was about something else? And so that, to me, as an investigator, is a very significant fact.”

The hearing also revealed an important distinction that Republicans are likely to maintain in trying to defend the President. Jim Risch, a Republican from Idaho, emphasized that Trump had used the word “hope” in discussing the prospect of killing the Flynn investigation. Trying to get Comey to acknowledge that this was short of a direct order, Risch asked if Comey knew of anyone who had ever been prosecuted “for hoping something.”

Comey replied, “I took it as a direction. I took it as, ‘This is what he wants me to do.’ ”

It is a remarkable measure of where the President stands, less than five months into his term: his party is seeking to defend him on the basis that his secret plea to the F.B.I. director, to abandon an investigation of a friend, did not rise to the level of an explicit order.

House Speaker Paul Ryan, in a separate appearance shortly after Comey’s comments, tried to define downward what the President is expected to understand about the law. He told reporters that Trump “wasn’t steeped in the long-running protocols” of what is appropriate in terms of discussions with the F.B.I. director. As a bulwark against disgrace, it’s a narrow spit of dry land—the “stupidity defense,” as Nicolle Wallace, a Republican commentator who is critical of the President, said on NBC.

After the hearing, Trump’s personal lawyer Marc Kasowitz e-mailed reporters a statement that effectively accused Comey of lying under oath: “The President also never told Mr. Comey, ‘I need loyalty, I expect loyalty’ in form or substance.” The statement also said that Trump never “directed or suggested” that Comey stop any investigation.

Wittingly or not, Trump’s team was setting up a public showdown of credibility between the President and his former F.B.I. director. The statement is a remarkable measure of the Administration’s seclusion from the events that are engulfing it. Comey’s testimony revealed the encounters between a skilled, sometimes pedantic student of the Constitution and an amateur with no moral governor and no prudent counsel. Whether or not Trump can be charged, or convicted, of a statutory offense will depend on the arguments of lawyers on both sides, but it’s now clear that Trump used the office of the President in ways that some Republicans, such as Susan Collins, find unacceptable. Collins, of Maine, said, “The President never should have cleared the room, and he never should have asked you, as you reported, to let it go, to let the investigation go.”

Comey’s testimony may mark the moment when Trump’s biggest legal risk shifted from the nature of his campaign’s links to Russia to the nature of his own actions to prevent investigation of those links. As I’ve written, the path to impeachment is not a legal and judicial process; it’s a political process—defined by the judgment of members of Congress, who decide when the President has lost the confidence of the public, because of his judgment or his ability to tell the truth or his capacity to operate the levers of government. By any measure, Trump took a step in that direction today.