The House would be in uncharted waters if McCarthy is removed as speaker: A motion to vacate has never been used successfully.

But the chamber wouldn't be as paralyzed and chaotic as it was in January amid McCarthy's five-day, 15-ballot vote to win the gavel.

As part of a rule change after 9/11 to support the continuity of government, the speaker is required to deliver to the House clerk an ordered list of members who can act as speaker pro tempore in the event of a vacancy.

The person at the top of McCarthy's list will serve as interim speaker until a new one is elected.

If the motion to vacate is successful, the chamber could go right to another speaker vote -- and McCarthy's allies could put him forward again to reclaim his post.

But unlike in January, it's possible that the House could take up other business in the meantime: The chamber has a rules package, and the speaker pro tempore would have the authority to act as speaker until a new one is elected.

A roll call vote is underway on the motion to vacate, which will decide whether McCarthy will keep his gavel.

If successful, it will be a historic moment. The motion to vacate has only been used once before -- more than a century ago -- and failed.

It would take as few as five Republican defections to oust McCarthy as speaker, if all Democrats vote against him.

House Majority Leader Steve Scalise also stood up to support McCarthy during the debate on the motion to vacate, saying now isn't the time to stop the progress House Republicans have made.

"When we go back to January, as many people have, we knew that it was going to be a narrow majority," Scalise said. "We also knew it wasn't going to be easy. How many of us came here because we thought this job was going to be easy?"

Scalise continued, "One thing we did know is that if we were going to finally start confronting the problems that had been ignored for years and years and years, we had to change the way this place worked. And one thing Speaker McCarthy embraced from Day 1 is to start making those kind of changes to this institution -- opening up the process, allowing members to be more engaged, having amendments come to the floor, single-subject bills, doing appropriations bills."

"Speaker McCarthy has been leading at the top of the level to make sure we have the tools to do our jobs," he added.

Gaetz said earlier this week he'd support Scalise for speaker and believed other Republicans would, too, telling reporters he thinks "very highly" of the No. 2 House Republican.

During the debate, after initially introducing speeches from a few other Republicans who are backing his motion, including Andy Biggs and Bob Good, Gaetz has since used his time to personally respond to each of the pro-McCarthy speeches being made by other lawmakers.

Those responses have sometimes been as brief as simply disputing the common refrain from McCarthy's backers: that moving to oust him is a short-sighted and punitive act that won't help the conference accomplish its goals.

Not so, Gaetz has continually said as he repeats his argument: McCarthy must go.

-ABC News' Adam Carlson

From the moment McCarthy was elected speaker, it was clear that his ability to keep that job could depend on Democrats.

To get elected on that 15th ballot in January, McCarthy had to agree to a series of rules that made him a profoundly weak speaker -- including the provision allowing a single member of the House to call for a vote to oust him (the now-famous "motion to vacate"). But while it takes just one member to call for such a vote, it requires a majority to actually remove the speaker.

McCarthy could survive a rebellion from a handful of renegade Republicans -- but he could not survive such a rebellion if Democrats go along with it.

Gaetz has said he expected Democrats to save McCarthy. Many others did too.

As recently as Tuesday morning, House Minority Leader Hakeem Jeffries suggested a way out of this for McCarthy. In an interview on MSNBC, he said House rules should be changed so that a single member could not force a vote on removing the speaker. Jeffries could deliver the votes to save McCarthy's job and the votes to change the rules to make it impossible for Gaetz to pull off a stunt like this again.

But Democrats would have a price for doing that. What would McCarthy offer in return? An end to the impeachment inquiry in President Joe Biden and his family? A guarantee that McCarthy honors the deal he struck on spending with the White House? Power sharing?

McCarthy has developed a cordial relationship with Jeffries, but he signaled no willingness to give Democrats anything in exchange for keeping the job.

In fact, McCarthy's actions have made it virtually impossible for Jeffries to throw him a lifeline. The speaker unilaterally launched the impeachment inquiry and just Saturday infuriated Jeffries by attempting to jam Democrats on a bill to keep the government funded for 45 days.

Back in January -- and again last month on impeachment -- McCarthy showed he was willing to do almost anything to appease his right-wing critics to get and keep his job. That worked for a while -- but it has made it virtually impossible to get Democrats to save him now.

-ABC News Chief Washington Correspondent Jonathan Karl