But Mr. Comey’s testimony on Thursday before the Senate Intelligence Committee sharpened obstruction-of-justice questions while refocusing them on an earlier conversation Mr. Comey said he had with Mr. Trump on Feb. 14 about an F.B.I. criminal investigation of Michael T. Flynn, Mr. Trump’s former national security adviser.
Specifically, according to Mr. Comey’s testimony, after he met that day with Mr. Trump and others in the Oval Office, Mr. Trump ordered all the other officials out of the room — twice reiterating to lingerers, Attorney General Jeff Sessions and Mr. Trump’s son-in-law and adviser, Jared Kushner, that they were to leave. Then, Mr. Comey said, Mr. Trump brought up Mr. Flynn, calling him a “good guy” and saying, “I hope you can let this go.”
“I took it as a direction,” Mr. Comey testified Thursday. “I mean, the president of the United States, with me alone, saying, ‘I hope this’ — I took it as, This is what he wants me to do.”
Senator Jim Risch, Republican of Idaho, responded to Mr. Comey, “You may have taken it as a direction, but that’s not what he said.” Mr. Risch also suggested that no one had ever been prosecuted for obstruction of justice based on saying, “I hope.”
In fact, there have been several cases showing it is possible to obstruct justice using those words. In a 2008 case involving a man who had pleaded guilty to bank robbery, the Court of Appeals for the Eighth Circuit, in St. Louis, upheld a judge’s decision to impose a longer sentence because the man had also obstructed justice by telling his girlfriend, a potential witness, “I hope and pray to God you didn’t say anything about a weapon.”
“We have examples all the time in criminal law of people saying things only slightly subtly, where everyone understands what is meant — ‘Nice pair of legs you got there; shame if something happened to them,’” Mr. Buell said.
In a statement shortly after the hearing, Mr. Trump’s personal lawyer, Marc E. Kasowitz, acknowledged that Mr. Trump had asked Mr. Comey about Mr. Flynn and had said that the former national security adviser was a “good guy.” But he denied that Mr. Trump had said or implied anything to Mr. Comey about stopping the investigation.
“The president never, in form or substance, directed or suggested that Mr. Comey stop investigating anyone, including suggesting that Mr. Comey ‘let Flynn go,’” Mr. Kasowitz said.
Still, Renato Mariotti, a former federal prosecutor who is now a white-collar defense attorney and a partner at Thompson Coburn, said that the level of detail Mr. Comey offered — in particular, the accumulation of nuggets about how Mr. Trump cleared the room, showing how important it was to the president to have no one else present — was “stronger than what we heard before” as evidence of nefarious intent.
But defense lawyers could still argue for a benign interpretation, Mr. Mariotti said, and he suggested that while the publicly available evidence was sufficient for a United States attorney’s office to consider bringing an obstruction case if it involved a business executive or a low-level political figure, the higher stakes of going after a president would give prosecutors pause. They would recognize, he said, “that it’s a hard case and you might lose.”
“Based only on what we know now in public, a reasonable prosecutor might bring this case against an ordinary person,” he said. “But a prudent prosecutor would want more facts before bringing this case against a president.”
The Justice Department has not said whether the special counsel, Robert S. Mueller III, is looking into any obstruction by Mr. Trump in addition to his mandate to examine Russia’s interference in the 2016 election and possible coordination by Trump campaign associates. But Mr. Comey said on Thursday that he had given his memos recounting conversations with Mr. Trump to Mr. Mueller, which would appear only to be relevant to an obstruction inquiry.
Julie O’Sullivan, a former federal prosecutor who now teaches white-collar criminal law at Georgetown University, said there may yet be more to learn from Mr. Flynn that could connect the Feb. 14 episode to the broader legal questions raised by the Russia investigation. If Mr. Trump did ask Mr. Comey to drop the Flynn case, she asked why he would do so.
“My supposition as a prosecutor would be that Flynn has something on the president,” she added. “You will recall that Flynn asked for immunity, saying that he has a story to tell. Perhaps the president believed that ending the investigation meant that Flynn would never tell that story. I assume investigators will be delving into this, as it is clearly relevant to whether the president acted with a corrupt intent in trying to derail the Flynn investigation.”