The Republicans’ majority in the House shrank even more this week after Democrats flipped one of the last outstanding races in the country, giving GOP lawmakers little wiggle room heading into next year.

Since Election Day, Republicans have touted a mandate bestowed upon them by voters to push ahead with a Trump-dominated agenda, but President-elect Donald Trump’s siphoning of lawmakers from the lower chamber has eaten away at an already thin majority.

His nominees from the House, which Speaker Mike Johnson has pleaded with him to stop, coupled with Democrats’ gaining seats, could set up a rocky path for Mr. Trump’s legislative agenda in the lower chamber.

Democrat Derek Tran flipped Rep. Michelle Steel’s seat in California’s 45th district on Wednesday, setting the current margin at 220 Republican seats and 214 Democratic seats.

Democrats also are poised to flip the last undecided House race in the country for Rep. John Duarte’s seat in California’s 13th District, where the incumbent now trails Democratic challenger Adam Gray by just more than 100 votes.

Rep. Marjorie Taylor Greene, Georgia Republican, on Wednesday called foul on the nation’s last remaining House race, saying that Democrats were “stealing another House seat.”

“Congressman John Duarte was winning but after 22 days of counting ballots he is now losing by 105 votes,” Ms. Greene said on X.

If Mr. Gray’s lead holds, his victory would give Democrats 215 seats. Republicans will have an even thinner margin, however, once the next Congress kicks off.

The race to install Mr. Trump’s agenda begins Jan. 20, when he is set to be inaugurated into his second term in the White House and officially kick off his first 100 days in office. The president-elect has already yanked three members from the House to be in his administration: former Rep. Matt Gaetz of Florida, and Reps. Mike Waltz of Florida and Elise Stefanik of New York. Their absences would leave Republicans with just 217 seats in the upcoming Congress.

Mr. Gaetz already resigned from the current Congress and said that he has no plans to return for the upcoming session after he withdrew his nomination as attorney general, while Mr. Waltz is set to resign on Inauguration Day to become Mr. Trump’s national security adviser.

Special elections to fill their seats will not be held until April 1, meaning the House GOP will be down at least two lawmakers for most of the president-elect’s first 100 days.

Ms. Stefanik has not announced when she will leave Congress as she takes on the role as U.S. ambassador to the U.N., but her exit will likely come early next year. Once she does vacate her seat, New York Gov. Kathy Hochul, a Democrat, will have 90 days to set a special election to fill the seat in the state’s 21st Congressional District.

Holding a 217-215 majority means Mr. Johnson has no room for error, as just one detractor or absent Republican could kill a bill. That mathematical reality could have oversized consequences on key items in Mr. Trump’s agenda, such as extending his 2017 tax cuts that are set to expire next year.

Mr. Johnson, Louisiana Republican, recognized before lawmakers left for Thanksgiving the consequences of an even tighter majority.

“Every single vote will count, because if someone gets ill or has a car accident or a late flight on their plane, then it affects the votes on the floor,” he said.