Hours after the US Capitol was besieged by a mob of President Donald Trump supporters on Wednesday, some lawmakers, members of the cabinet and a US trade group have called on their colleagues to initiate 25th Amendment proceedings over the role they say Trump played in inciting the illegal siege.

Talks of impeaching Trump also began circulating in the early hours of the Capitol breach as Congress members were shuttled into the safety of an undisclosed location under the protection of Capitol police, with Rep. Ilhan Omar tweeting that she was drawing up Articles of Impeachment. If passed, the action would represent Trump’s second impeachment by Congress during his elected term, with just shy of two weeks until President-elect Joe Biden’s Jan. 20 inauguration. It would also be the second time lawmakers have discussed invoking the 25th Amendment.

Ahead of the impeachment talks, officials in the Trump administration said that Vice President Mike Pence had approved an order to deploy the Washington DC National Guard. This authority is traditionally reserved for the president alone.

After a statement from Acting Secretary of Defense Christopher Miller saying he spoke with Pence, multiple House Democrats began urging Pence toward action. If the 25th Amendment were enacted, Pence would assume the presidency.

So, what does the 25th Amendment say? Why is it so important, and how likely is it to be enacted? Here’s what you need to know.

Read more: Mob storms Capitol as Facebook, Twitter roles come under fire

What is the 25th Amendment?

In the most simple explanation, the 25th Amendment contains four sections and pertains to the president’s ability to perform the duties of the presidency and what happens in the event that the commander in chief can no longer do his or her job. The amendment empowers the vice president to temporarily become president, enabling a smooth transition of power in an emergency.

The amendment also says the president can nominate a vice president if there’s a vacancy.

The part of the 25th Amendment now under discussion generally relates to Section 4, which would allow the vice president and a majority of either the presidents cabinet or the members of Congress to declare in writing to the Senate president pro tempore and House speaker that the sitting president is unable to perform the duties of the office. This immediately makes the vice president the acting president.

The current president can push back on this effort by the vice president and Congress, however, declaring him or herself fit for office in official writing. From there, the vice president and those supporting impeachment have four days to disagree, or the sitting president resumes the presidency. If they disagree, Congress can settle the matter with a vote.

Why are lawmakers urging Pence to use the amendment?

As the events on Capitol Hill unfolded, lawmakers, Trump’s business supporters and constituents began calling for the president’s removal from office. After speaking at a nearby rally, Trump told his gathered supporters that “[w]e will never give up, we will never concede.” Trump’s tweets, many of which have now been deleted or blocked, later continued to instigate the crowd.

“We’re going to walk down Pennsylvania Avenue … and we’re going to the Capitol,” Trump said at the rally. Afterward, supporters marched to the Capitol, where they later broke past barricades and entered the building. Hours passed as constituents and lawmakers urged Trump to call for the mob to stand down.

Trump eventually gave a brief taped statement telling the rioters to go home, calling them “special people,” while continuing to circulate false claims of voter fraud, as he has done for months.

As night fell, social media cracked down on Trump with Twitter flagging and deleting multiple tweets. In addition, the site slapped Trump with a temporary suspension. Facebook and Instagram followed suit.

“We’ve assessed two policy violations against President Trump’s Page which will result in a 24-hour feature block, meaning he will lose the ability to post on the platform during that time,” Facebook tweeted.

When else was the 25th Amendment discussed regarding Trump?

This isn’t the first time the 25th Amendment has been mentioned during Trump’s presidency.

On Oct. 1, Trump announced on Twitter that he and first lady Melania Trump tested positive for COVID-19. Shortly after, he was hospitalized, given oxygen and treated with a powerful medication known to have “euphoric” side effects.

Following his diagnosis, House Speaker Nancy Pelosi introduced legislation that would allow Congress to enact the 25th Amendment if Trump became incapacitated, although she insisted at the time that the legislation wasn’t specifically aimed at Trump.

When released from the Walter Reed National Military Medical Center, Trump, while still under physician supervision and the steroid dexamethasone, abruptly stopped stimulus check negotiations, only to reinstate them hours later, and offer a stimulus package that ultimately fizzled out. At the time, congressional Democrats discussed invoking the 25th Amendment, but didn’t bring it to a vote.

Late Wednesday, the Democrats on the House Judiciary Committee wrote a letter to Pence, urging him to invoke the amendment.

“For the sake of our democracy, we emphatically urge you to invoke the 25th Amendment and begin the process of removing President Trump from office,” according to the letter, lead by Reps. David Cicilline (Rhode Island) and Ted Lieu (California).

How would the 25th Amendment be used?

Section 4 of the 25th Amendment says:

“Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.”

The “principal officers of the executive departments” refer to the 16 secretaries within the president’s cabinet. Pence would need to have eight or more to join with him in a written declaration regarding the president’s inability to complete his duties.

In the event that the 25th Amendment is used, it would mean Pence filling Trump’s vacated presidency and would also give Pence the ability to nominate a new vice president. Trump could declare to the House speaker and the pro tempore of the Senate that there is “no inability” for him to govern. At that point, it would be up to Congress to decide on the matter within 21 days, which would pass the Jan. 20 date when President-elect Joe Biden takes office.

Has it ever been used before?

Section 4, the portion largely referenced throughout the week, has never been enacted, only coming close once during an attempted assassination on President Ronald Reagan in 1981.

Congress approved the 25th Amendment in 1965. It was ratified and certified as an amendment the following year by President Lyndon Johnson.

The first usage of the other sections of the 25th Amendment was in 1973 when President Richard Nixon nominated Gerald Ford after Vice President Spiro Agnew resigned. The amendment was used once more when Nixon resigned and Ford assumed the presidency. Ford also used the amendment when he chose Nelson Rockefeller to fill the vice presidency.

Most recently, President George W. Bush twice invoked the 25th Amendment to temporarily transfer the powers of the presidency to Vice President Dick Cheney while Bush underwent colonoscopies — first in 2002 and then again in 2007 — under anesthesia. His father, then Vice President George H. W. Bush, was the recipient of 25th Amendment authority from President Ronald Regan in 1985.