Did President Donald Trump commit obstruction of justice when he reportedly asked then-FBI Director James Comey in February to end the agency’s investigation into Michael Flynn?

The question became critical for the White House this week, after the New York Times and other news organizations reported Tuesday that Comey wrote a memo detailing this request by Mr. Trump. The White House has denied that Trump urged Comey to drop the FBI’s probe into Flynn, who resigned as national security adviser in February over his contacts with a Russian official.

Comey’s memo — coming on the heels of his dismissal May 9 — raised new concerns over whether Trump intended to thwart the FBI investigation into his campaign’s ties with Russia. Democrats have ramped up calls for a special counsel or independent commission.

READ MORE: The ‘special prosecutor’ Democrats want no longer exists

“At best, President Trump has committed a grave abuse of executive power,” House Minority Leader Nancy Pelosi, D-Ca., said in a statement Tuesday. “At worst, he has obstructed justice.”

But proving obstruction of justice is difficult. Different sets of criteria are used to determine whether a president’s alleged obstruction of justice is a federal criminal offense, and whether it’s an impeachable offense. Here’s a guide to the issue:

Obstruction of justice under federal statute

There are a number of relevant federal statutes, all under Title 18 of the United States Code, that Trump could have possibly violated. An obstruction of justice charge would need to meet three key requirements under the law, Joshua Matz, an attorney and constitutional law expert, wrote in an email: “1) there must have been a pending federal investigation or proceeding; 2) the defendant must have known about it; and 3) the defendant must have corruptly endeavored to influence, obstruct, or impede the investigation or proceeding.”

Was Trump acting corruptly?

That’s a tricky question to answer. Proving that Trump acted corruptly with the specific intent of impeding the FBI’s investigation isn’t easy, said Michael Gerhardt, the National Constitution Center’s scholar in residence.

“It’s hard to do generally as part of a criminal case, because you have to show bad intent beyond a reasonable doubt,” said Gerhardt, a professor at the University of North Carolina at Chapel Hill.

“We can never know what [Trump] was actually thinking,” said Julie O’Sullivan, a professor at the Georgetown University Law Center.

But there are several ways a prosecutor could build a case, O’Sullivan said. Prosecutors could question Trump’s top aides, or call Trump before a grand jury. (You might remember that independent counsel Kenneth Starr called President Bill Clinton to testify before a grand jury during his probe into the Clinton White House). Prosecutors could also look at circumstantial evidence, such as Trump’s decision to fire Flynn and Comey and the timing of those dismissals, O’Sullivan said.

For example, Trump’s conversation with Comey, along with the fact that he fired Comey just days after the FBI director asked for more resources for his Russia probe, could look bad for Trump, O’Sullivan said.

“A good prosecutor could probably put together an incredible case,” O’Sullivan said. She added: “There’s enough to warrant an investigation.”

What stands in the way of a criminal case?

It’s unlikely the Department of Justice, led by Trump ally Attorney General Jeff Sessions, would rush to launch a federal case against the president. And some experts have argued that there’s still no damning evidence.

Media reports of Comey’s memo remain incomplete, leaving a lot of unknowns about what exactly was said in the meeting between him and Trump. In a letter to the FBI Tuesday, Rep. Jason Chaffetz, chairman of the House Oversight and Government Committee, demanded a copy of Comey’s memo.

With so many impediments to a federal charge, attention has turned to potential impeachment proceedings.

How would impeachment proceedings work?

Impeachment proceedings go through Congress. The House initiates, and requires a majority vote. The Senate tries the proceedings, and needs a two-thirds vote for conviction. “Obstruction of justice” has motivated past congressional attempts to impeach presidents: it was the first article of impeachment leveled against President Richard Nixon (before he resigned), and it was the third article of impeachment for Clinton.

But impeachment proceedings operate differently than federal court cases. “It depends on what the House of Representatives and two-thirds of the Senate believe is an impeachable offense,” O’Sullivan said. “It’s not a criminal trial, it’s a public trial of accountability.”

The Senate could consider broader questions of intent and purpose, not just the high legal bar of “corrupt” intent. While Trump’s habits and history would be on the table, the Senate could also choose to ignore his actions.

And of course, coloring all of this is the fact that impeachment proceedings are innately political. What Sen. Ted Cruz, R-Texas, thinks about Trump’s guiltiness, for example, could differ widely from the views held by Sen. Al Franken, D-Minn.

“The party composition of Congress is” important,” Gerhardt said. “The Republican control of the House and Senate [would likely] slow down the process.”