A memo detailing President Donald Trump's request to shut down an FBI investigation of his ousted national security adviser is a powerful piece of evidence that could be used to build an obstruction of justice case against him.
But criminal charges of interfering with an investigation are difficult in ordinary circumstances, several former federal prosecutors cautioned Wednesday. And it's an open question whether a sitting president can even face criminal charges, or whether attempts to hold him accountable for wrongdoing can only proceed through impeachment.
The White House has denied fired FBI Director James Comey's account of a conversation he had with Trump in February, recorded in a memo seen by Comey associates. Still, to a prosecutor building an obstruction of justice case, the facts are compelling.
First, Comey says the president asked for his loyalty. Then he asked Comey to back off the FBI's investigation of former National Security Adviser Michael Flynn, according to Comey's memo. Then Trump fired Comey, declaring in a television interview that it was at least partly because he was bothered by the FBI's probe of potential coordination between Russia and his presidential campaign.
"I'm hard pressed to find a prosecutor who would just look at this and say, OK, just move along," said Jeff Cramer, a former federal prosecutor. "It's not a two-foot putt, but is it a viable prosecution? Sure. But not against a president."
Whether a sitting president can be indicted for a crime is a question that is not answered by either the Constitution or by Supreme Court cases, but many legal scholars believe impeachment is the only way to hold a president accountable for misconduct. Justice Department lawyers under presidents Richard Nixon and Bill Clinton also concluded a president can't be indicted because it would interfere with his constitutionally assigned duties.
Eric Freedman, a Hofstra University law professor, has long believed that a president can be prosecuted while in office. "It's a disputed constitutional issue. We know specifically that the framers argued about this," Freedman said. "They were undecided."
Even if possible, a criminal prosecution would have its hurdles. Obstruction of justice charges often come down to whether prosecutors can show a person "corruptly" intended to interfere with an investigation.
"Since we can't read people's minds, proving intent is very difficult," said Carole Rendon, who served as the chief federal prosecutor in Cleveland until March. And Trump could argue he was not corrupt in raising an issue of concern for Flynn, a longtime associate.
Proving Trump's mental state would be a tall task, but prosecutors could have some clues.
For one, he asked Vice President Mike Pence and Attorney General Jeff Sessions to leave the room before speaking privately with Comey about Flynn, according to a Comey associate. And in firing Comey, Trump claimed the FBI director told him three times he was not under investigation, which could be a sign that they were repeatedly having such conversations, said Daniel Petalas, a defense attorney who formerly worked in the Justice Department's Public Integrity Section.
"If he's pressuring repeatedly and won't take no for an answer, that becomes evidence of intent," Petalas said.
Comey's response could also be an indication.
"If in fact it's true that the FBI director immediately wrote a memo to the file, it indicates that he thought something was amiss," Rendon said.
The memo itself would be "significant evidence," because Comey has a "sterling reputation for truthfulness," said Paul Charlton, another former federal prosecutor. "I don't know that you could say the same about the president."
Both presidents who were subjected to impeachment proceedings, Clinton and Nixon, were accused of obstructing justice. Yet impeachment would not be without its own difficulties, Petalas said, because the House and Senate decide for themselves whether the standard is met.
It takes a majority of the House to impeach a president and a two-thirds vote of the Senate to remove him from office. Republicans control both houses, as well as the presidency.
Nixon resigned before the Democratic-led House could vote on impeachment and Clinton, while impeached by a House run by Republicans, survived a Senate trial.
In theory, prosecutors can pursue charges against a president after he leaves office. President Gerald Ford pardoned Nixon in 1974 to spare him and the country from having to re-live the Watergate scandal in a historic trial of a former president.
Days before he left office, Clinton reached a deal with independent counsel Robert Ray that spared Clinton possible indictment after leaving office. Clinton acknowledged that he had made false statements under oath about his relationship with White House intern Monica Lewinsky and surrendered his law license in Arkansas.