Revelations that President Donald Trump asked then-FBI Director James Comey to end an investigation before firing him have sparked a debate about whether or not Trump’s actions were an obstruction of justice.

Trump wanted the FBI to stop the probe into former national security adviser Michael Flynn and his ties to Russia, according to reports citing sources who saw a memo written by Comey.

An obstruction of justice, or an attempt to influence, obstruct or impede an official investigation, is considered a federal offense and possibly an impeachable offense, according to experts and lawmakers.

“Asking FBI to drop an investigation is obstruction of justice. Obstruction of justice is an impeachable offense,” Rep. Ted Deutch (D-FL) wrote on Twitter.

But proving obstruction of justice is not an easy task, and neither is removing a president from office.

Here’s a look at the steps required to remove a sitting president:

Step 1: House of Representatives vote

Articles of impeachment, or a charge against a president for violation of the Constitution, must first be brought up in the House of Representatives. Typically, the Judiciary Committee starts with an investigation and then votes on whether or not to recommend specific articles of impeachment, American University professor Allan Lichtman writes in his book “The Case for Impeachment.”

The House must then vote on the articles of impeachment, and a simple majority is needed to ratify them. If the vote passes, the president is impeached, but not yet convicted.

An impeachment by the House is a step toward removing a president, but that alone cannot force him to step down.

Step 2: The trial 

A trial is held in the Senate to determine if the president will be convicted of violating the Constitution. The House appoints the prosecutors and the president chooses lawyers for the defense. The president is not required to appear at the trial.

Step 3: The Senate vote

In order for a conviction to pass, the Senate must have a two-thirds majority. Only if that vote passes, would the president be forced to step down. 

The verdict of the House and the Senate is final. “There is no right of appeal or judicial review of their decisions,” Lichtman writes in the first chapter of “The Case for Impeachment.” 

What can a president be impeached for?

The Constitution says a president can be impeached if he or she has committed “treason, bribery, or other high crimes and misdemeanors.” What “high crimes and misdemeanors” qualify for impeachment is left up to interpretation. Ultimately, the decision is left up to Congress. 

“Decisions in whether to impeach a president turn on the wisdom of Congress and do not require proof of a specific indictable crime under either federal or state law,” Lichtman says in his book. 

Only two presidents in American history have been impeached — Bill Clinton and Andrew Johnson — and neither were convicted in the Senate. Richard Nixon resigned before the House could vote on the articles of impeachment.

Johnson was impeached because he fired his war secretary, Edwin Statnton, without Senate approval, despite a law at the time barring that authority. 

Both Clinton and Nixon, had obstruction of justice accusations brought against them.

“Charges of obstruction of justice have been a staple of the last two substantial attempts to impeach a president,” William Yeomans, fellow in law and government at American University, wrote recently in column for “The Hill.”

Nixon was accused of obstructing justice for interfering with the FBI Watergate investigation, and Clinton was accused of impeding justice by lying about and covering up his relationship with Monica Lewinsky.