President Trump speaks to reporters while walking on the South Lawn of the White House. (Brendan Smialowski/AFP)

Since the beginning of the Russia investigation, there has been a recurring narrative that President Trump could shut it down whenever he wanted. It’s so much a part of the conversation that Sen. Jeff Flake (R-Ariz.), in his final act in Congress, has blocked every Trump judicial nominee until there’s a vote on legislation to protect Robert S. Mueller III and his special counsel inquiry from any interference by Trump.

Wednesday’s sentencing of longtime Trump lawyer Michael Cohen, who in his final plea for leniency said of the president, Time and time again, I felt it was my duty to cover up his dirty deeds,” must make it seem to Trump and his inner circle that the walls are closing in. If there were ever a time for Trump to seek a way to stop the careening boulder headed for his presidency, his business and his family, that time would be now.

But to continue the metaphor, the boulder may be unstoppable at this point.

It was never going to be easy for Trump to stop the Russia investigation even at its conception, said Jens David Ohlin, vice dean of Cornell Law School. But with Democrats taking control of the House and the prosecutors beginning to release documents, the president is effectively out of options. Trying to end it now, Ohlin said, “would precipitate a political and legal crisis.”

“Trump’s options radically diminish after the new year,” Ohlin said. “I think he has no good options, or he has no perfect options, and all of the various options come with tremendous cost.”

Trump could, hypothetically, direct Matthew G. Whitaker, the acting attorney general, to fire Mueller, reframe Mueller’s mandate for the investigation, constrain Mueller’s budget or even bury the report once it lands. But any of those actions would result in Democrats subpoenaing documents or the report itself, and almost certainly beginning the impeachment process, Ohlin said.

An article published in Slate on Wednesday outlined another safeguard for the Mueller investigation: the grand jury. The author suggests that if Mueller were fired, and if no one was appointed to replace him to finish the job, then Judge Beryl Howell, who is overseeing the grand jury, should be able to appoint a new prosecutor, although there’s no real precedent for doing so.

Former FBI director James B. Comey, who was summoned to testify in the House last week regarding how the agency handled the investigation of Hillary Clinton’s emails, was asked by a Democratic lawmaker what would happen if Trump fired Mueller.

“As an informed outsider, I think that it would — you’d almost have to fire everyone in the FBI and the Justice Department to derail the relevant investigations,” Comey said.

Ohlin said that at this point, Trump, though unpredictable, is unlikely to try to intervene, knowing full well the ramifications of doing so. Even if he did, he probably wouldn’t be able to shield himself or his family from whatever Mueller’s team uncovers, Ohlin said.

“He’s been advised by his counsel that he has to let this play out,” Ohlin said. “He’s going to let the report be issued because if he does something now, it’s going to trigger some kind of backlash that would make the situation for him worse.”