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

Now-former Speaker McCarthy addressed reporters from Capitol Hill on Tuesday night where he looked back at his years in Republican leadership, touted his record -- and struck an optimistic if ambiguous note about his future.

"I believe I can continue to fight," McCarthy said, while noting that he will not run again for speaker after he lost a historic vote on the motion to vacate earlier in the day.

"My goals have not changed. My ability to fight is just in a different form," he said, later adding, "I'll never give up on the American people. That doesn't mean I have to be speaker."

McCarthy also swiped at the conservative rebels who ousted him from his role -- stressing that they were just 4% of the GOP conference -- and claimed that Gaetz was acting out of a personal sense of grievance in pushing him out, which Gaetz denies.

Despite the defeat he suffered from within his own party, McCarthy sounded largely sanguine about his political career.

He contended that the Democratic minority deciding to vote against him had made a "political decision" that undercut the ability of the House to govern.

"My fear is the institution fell today," he said.

Still, in largely valedictory remarks, he pointed back to work to cut government spending, expand his party's base and increase their majority in Congress, including by electing more women and minorities.

"I feel fortunate to have served the American people," he insisted. "I leave the speakership with a sense of pride, accomplishment and, yes, optimism."

-ABC News' Adam Carlson