How does the 25th Amendment work?
What is obstruction of justice?
Special counsel Robert Mueller's probe into whether President Trump's campaign colluded with Russia to influence the outcome of the 2016 presidential election has expanded to whether Trump himself obstructed justice.
But what exactly does that mean?
Obstruction of justice is the federal crime of "corruptly or by threat, or force" trying to influence, obstruct, influence or impede the due process of justice.
Legal experts say there is a multitude of ways that obstruction can be committed, including destroying or tampering with evidence, intimidating witnesses or trying to cover up a crime.
When it comes to Trump, Mueller is investigating whether the president tried to impede the Russia investigation and influence those involved.
"It's not about an underlying crime," said Josh Blackman, an associate professor of law at the South Texas College of Law in Houston. "It's not about whether Trump colluded with Russia, it's about whether he interfered with or obstructed the investigation of that charge."
But to be charged with obstruction of justice, a person has to have acted with a "corrupt intent," which makes the crime that much harder to prove.
The statute, which dates back to 1789, was purposely written in broad terms.
"It's a very general and vague concept and at the time it was written the idea was 'we'll know it when we see it,' " said Rory Little, a law professor at the UC Hastings College of the Law in San Francisco, who served as an associate deputy attorney general under U.S. Attorney General Janet Reno.
"It was an intentionally open-ended concept because the varieties of human experience will exceed the ability of language to capture them," he added.
Obstruction of justice is a felony charge and penalties can include a fine, up to five years in prison or both.
Some say Trump's firing of FBI Director James Comey is clear evidence he engaged in obstruction of justice. Comey testified to Congress that Trump had pressured him in an Oval Office meeting to end the investigation into Trump's former national security adviser Michael Flynn, who ultimately pleaded guilty to lying to FBI agents in the course of the Russia investigation.
Trump denies making that request of Comey, who he has blasted as a "leaker."
The president's private lawyer, John Dowd, has also claimed that the president can't be found guilty of obstruction of justice.
In a December interview with Axios, he said the "President cannot obstruct justice because he is the chief law enforcement officer under [the Constitution's Article II] and has every right to express his view of any case."
Whether a sitting president can or can't be charged with a crime is still unsettled law, law professors say.
Legal scholars say it's more likely that Mueller would release evidence that Trump obstructed justice and let the political process take it from there.
"There's a slim chance there will be an indictment so to focus on the statutory crime is a little misleading," said William Yeomans, a fellow at the liberal Alliance for Justice, former law professor and 26-year veteran of the Department of Justice.
"What we're really talking about is whether the president has committed an impeachable offense."
And obstruction of justice, Yeomans argued, is a well-established ground for impeachment.
Former President Nixon and Clinton were both accused of having prevented, obstructed and impeded the administration of justice.
In Nixon's case, one of the articles of impeachment approved by the House Judiciary Committee alleged he tried to obstruct the investigation of the Watergate break-in and cover up other unlawful activities. Nixon resigned before he could be impeached and then removed from office.
Clinton was impeached by the House for lying under oath and obstructing justice to cover up his affair with Monica Lewinsky, but was not convicted or removed from office by the Senate.
Some legal scholars, including Blackman, argue that Trump's decision to fire Comey can't be the basis for an obstruction of justice charge because the president has the constitutional authority to fire the head of the FBI.
In a 2000 opinion, the Justice Department said "the indictment or criminal prosecution of a sitting president would unconstitutionally undermine the capacity of the executive branch to perform its constitutionally assigned functions."
Yeomans said Mueller and Deputy Attorney General Rod Rosenstein, who is overseeing the special counsel's investigation, are likely to follow the agency opinion. But he argued the evidence against Trump is clear.
"At this point I don't think it's that complicated," he said. "I think Trump has so clearly crossed the line of obstruction that if a case were to be brought, it would be perfectly justified and there is a strong precedent that what he has done is an impeachable offense."