ABC projects that Democrat Adam Gray will win the race for California's 13th Congressional District, unseating incumbent Republican John Duarte and flipping the final unresolved seat in the 2024 election.

With all 435 House races projected, ABC News estimates Republicans will hold 220 seats and Democrats 215 in the 119th Congress.

But it’s not clear how vacancies -- or, illness or other absences -- will impact the day-to-day division of power when the House convenes on Jan. 3.

President-elect Donald Trump initially tapped three House Republicans for positions in his upcoming administration: Florida Reps. Matt Gaetz and Mike Waltz, and New York Rep. Elise Stefanik. Gaetz has already resigned from Congress and withdrew last month from consideration to serve as President-elect Donald Trump's attorney general. And though he won reelection to his seat last month, he said he won't serve another term.

Republicans could have a 217-215 majority while their seats are vacant -- the narrowest GOP majority in history -- and special elections to fill those seats can take months to complete.

In this case, any single Republican can hold Johnson hostage: Losing just one Republican on a vote would result in a 216-216 tie.

The speaker acknowledged the thin majority, which could pose a challenge in passing Trump’s agenda.

“Well, just like we do every day here, we've developed an expertise in that. We know how to work with a small majority. That's our custom now,” Johnson said. “So, yes -- do the math. We can, we have nothing to spare. But all of our members know that we talked about that today, as we do constantly, that this is a team effort that we've got to all row in the same direction.”

But several elderly Democrats have missed votes recently, which could give Republicans a little more breathing room next year.

In California's 13th District, Duarte conceded to Gray on Tuesday, according to the Turlock Journal.

"I’m a citizen legislator, and I didn’t plan on being in Congress forever," Duarte told the newspaper. "But whenever I think I can make a difference, I’ll consider public service in different forms, including running for Congress again."

Gray released a victory statement on X Tuesday evening, extending his gratitude and saying the "final results confirm this district is ready for independent and accountable leadership that always puts the Valley's people ahead of partisan politics."

-ABC News' Marilyn Heck and Benjamin Siegel contributed to this report.