The 25th Amendment to the U.S. Constitution outlines the process for removing a sitting president from office. But what is the procedure it sets forth? And what could it mean for President Donald Trump? Here's what you need to know:

What is the 25th Amendment?

The 25th Amendment outlines steps to fill the presidency or vice presidency in the event that either official is unable to "discharge the powers and duties" of their office, whether that means the president was removed from office, died or resigned the position. Congress proposed the amendment in the aftermath of President John F. Kennedy's assassination in 1963. It was officially adopted in 1967.

When can it be used?

Politicians can invoke the amendment if they follow procedures outlined in the amendment's four sections. Those sections include:

Section 1: If the president is removed from office, dies or resigns, the vice president assumes the Oval Office.

Section 2: If there is a vacancy in the office of the vice president, the president nominates a person to fill the role. The vice president takes office upon confirmation by a majority vote of both houses of Congress.

Section 3: The president can temporarily transfer all of his presidential duties and powers to the vice president until he declares that he can once again discharge them.

Section 4: The vice president and the Cabinet can oust a president they determine is unfit. The vice president and a majority of the president's Cabinet members must agree the president can no longer perform his duties before they notify Congress. At that point, the vice president becomes president until the president counters, in writing, that he is fit to lead. The vice president and the president's Cabinet can challenge the president's assertion within four days, and Congress then has 21 days to decide who is right.

Has it been used before?

Yes – parts of it, anyway. The procedures were used during the Watergate scandal when Gerald Ford replaced Spiro Agnew as vice president and then replaced Richard Nixon as president, in line with the first and second sections. In another case, former President George W. Bush invoked the third section of the amendment when he underwent routine colonoscopy procedures, passing his presidential powers to then-Vice President Dick Cheney.

A president has never been ousted under Section 4. Some have argued that the amendment should have been invoked after an assassination attempt on Ronald Reagan in 1981 to allow the president more time to recover from his gunshot injuries. But the only other time officials are reported to have considered its use was after Howard Baker Jr. replaced Donald Regan as Reagan’s chief of staff and concerns abounded about the mental fitness of the president, who would later disclose a diagnosis of Alzheimer's disease.

What could it mean for President Donald Trump?

The potential use of the 25th Amendment's fourth section is controversial, with critics saying removing a president for political reasons under the section is a misinterpretation of the amendment.

Vice President Mike Pence and a majority of the members of the president's Cabinet would have to agree that Trump is unfit to execute the powers and responsibilities of his office – an agreement that could be difficult to reach. CBS News, however, aired an interview clip with former FBI Deputy Director Andrew McCabe in which McCabe said Justice Department officials were so taken aback by President Donald Trump's decision to fire FBI Director James Comey in 2017 that they discussed recruiting cabinet members to use the 25th Amendment to remove Trump from office.

And McCabe, in a memo obtained by The New York Times, wrote: "We discussed the president's capacity and the possibility he could be removed from office under the 25th Amendment." The deputy attorney general, Rod Rosenstein, reportedly said he'd look into it and added that he'd need a "majority or 8 of the 15 cabinet officials." He also suggested he may have the support of some of the members, according to the Times.

In response to the aired clip, Trump lashed out on Twitter. Congress, of course, also has the power to impeach the president for "treason, bribery, or other high crimes and misdemeanors."

