WASHINGTON — President Donald Trump’s assertion that he fired his former national security adviser, Michael Flynn, in part because he knew that Flynn had lied to the FBI about his conversations with the Russian ambassador — for which Flynn pleaded guilty Friday — has intensified accusations that the president committed obstruction of justice.

Critics of Trump have portrayed the statement as a confession that he knew Flynn had committed a crime — not just that Flynn had misled Vice President Mike Pence, the initial justification the White House gave for his firing — when he pressured James Comey, the FBI director at the time, to drop the investigation into Flynn, according to Comey’s testimony before Congress. Trump later fired Comey.

Trump has denied that he pressured Comey to drop the investigation. And one of Trump’s lawyers, John Dowd, has since said that he, not the president, drafted the tweet about firing Flynn because he lied to the FBI. Dowd has also claimed that the president, as a matter of constitutional law, cannot violate obstruction of justice statutes anyway.

Here is what you need to know to make sense of this.

— What is obstruction of justice?

Several federal statutes criminalise actions that impede official investigations. While some examples of illegal ways to thwart the justice system are specific — like killing a witness or destroying evidence — the law also includes broad, catchall prohibitions. For example, Sections 1503, 1505 and 1512 of Title 18 have variants of language making it a crime if someone corruptly “obstructs, influences or impedes any official proceeding.”

— Does Trump have authority to supervise law enforcement decisions and fire subordinates?

Yes. But courts have ruled that otherwise lawful acts can constitute obstruction of justice if done with corrupt intentions. In a 1998 case, for example, an appeals court upheld the conviction of a lawyer who had filed legal complaints and related motions against a government agent who was investigating an illegal gambling operation. The court ruled that the defendant’s “nominally litigation-related conduct” was unlawful because his real motive was “to safeguard his personal financial interests” in the corrupt enterprise.

Julie O’Sullivan, a former federal prosecutor who now teaches white-collar criminal law at Georgetown University, said the power relationship between a president and the FBI director could elevate a request to shut down a case into an act that amounts to impeding an official investigation.

— What is the argument that a president cannot commit obstruction of justice?

As a preliminary matter, there is no precedent for prosecuting a current or former president in criminal court for that offence. And several legal commentators have defended Trump by claiming that a president cannot be prosecuted for obstruction of justice because Article II of the Constitution gives the president authority to supervise and control the executive branch, including law enforcement decisions and personnel.

One version of this argument is that Trump cannot be charged in court with a crime because the judicial branch should not be in the position of second-guessing whether a president has properly exercised his constitutional authority, so the only remedy for abuse of power is impeachment and removal by Congress.

On Monday, Alan Dershowitz, a Harvard Law School professor and criminal defence attorney, went even further on Fox News’ Fox & Friends, suggesting that it might not even be a legitimate basis for impeachment. Trump endorsed that clip as a “much watch” on Twitter.

“I think if Congress ever were to charge him with obstruction of justice for exercising his constitutional authority under Article II, we’d have a constitutional crisis,” Dershowitz said. “You cannot charge a president with obstruction of justice for exercising his constitutional power to fire Comey and to tell the Justice Department who to investigate and who not to investigate.”

— What is the argument that a president can commit obstruction of justice?

Both US presidents who were subjected to impeachment proceedings in the last century — Bill Clinton in 1998, for suborning perjured testimony, and Richard M. Nixon in 1974, for trying to stop the Watergate investigation — were accused of obstruction of justice by Congress.

Other legal commentators maintained that the Constitution does not give presidents the power to abuse their supervisory powers by exercising them for an improper purpose — like to cover up a crime.

“This becomes a kind of ‘the king can do no harm’ argument, which just isn’t consistent with American criminal law or constitutional law,” said Samuel Buell, a Duke Law School professor and former federal prosecutor.

— Does Trump’s tweet strengthen a hypothetical obstruction case?

Obstruction of justice cases often come down to whether prosecutors can prove defendants’ mental state when they committed the act. It is not enough to show that a defendant knew the act would have a side consequence of impeding an investigation; achieving that obstruction has to have been the specific intention. The question is whether Trump’s tweet about why he fired Flynn — even if it could be shown that despite Dowd’s account, Trump did write it or approve it — bolsters that case.

Buell said it did, but he also suggested that critics of Trump were overreacting to Trump’s Twitter post — because, he said, if one believes Comey’s account, then there was already significant evidence that Trump was acting with an improper purpose.

“The conversation with Comey back in the spring is enough to show that Trump thought Flynn had at least serious potential for criminal exposure,” he said. “So if he also knew more detail about Flynn having lied in the FBI interview, it just adds weight. It doesn’t suddenly put everything in a different light.”

— What about impeachment?

“Asking FBI to drop an investigation is obstruction of justice,” Rep. Ted Deutch, D-Fla., said on Twitter after the initial report of the Oval Office conversation about Flynn. “Obstruction of justice is an impeachable offense.”

While it can be a murky task in court to interpret the obstruction statutes, said David Sklansky, a former federal prosecutor who teaches at Stanford, impeachment proceedings are different. They are a “quasi-judicial, quasi-political process,” he noted; the House and Senate determine for themselves whether the standards are met.

In other words, as a practical matter, the Constitution’s standards for impeachment and removal of a president — if he has committed “treason, bribery, or other high crimes and misdemeanors” — are met by anything that a majority of the House and two-thirds of the Senate are willing to vote for.

That makes prognostication an exercise in vote counting, not legal analysis. As presently constituted, the Republican-controlled Congress has shown little appetite for removing Trump from office.

Charlie Savage/The New York Times