The House of Representatives is back in session for four weeks straight on Monday after a brief recess — and for House Speaker Mike Johnson, R-La., it will be the start of a legislative sprint with one fewer reliable House GOP vote.

Rep. Mike Gallagher, R-Wis., who was regarded as a rising Republican star, shocked even some of his colleagues when he announced last month that he would leave Congress before the end of his term. The House announced his official departure on Friday.

Due to the timing of his retirement, his seat will not be filled until the next congressional term in January 2025.

Johnson’s public remarks and even leadership decisions have reflected that he’s keenly aware of the historically slim two-seat margin he’s been dealing with. 

It’s likely to get even smaller sometime over the next several weeks — at least for a time. 

The special election to replace retired Rep. Brian Higgins, D-N.Y., in New York’s 26th Congressional District is Tuesday. The heavily urban seat skews in favor of Democrats; President Biden won the Buffalo-area district by nearly 30 points in 2020.

House GOP leaders are expected to get some relief in late May, when two Republicans running to replace ex-Speaker Kevin McCarthy, R-Calif., face-off in a special runoff election. McCarthy left the House at the end of last year, but no candidate was able to secure a majority vote in the March race to replace him — by California state law, it triggers a runoff.

But if Higgins’ old seat stays in Democratic hands, it’s likely Johnson will have to navigate at least part of this four-week stint with just a one-vote majority. That means he’d only be able to lose one Republican lawmaker on any party-line vote.

A House GOP aide who spoke with Fox News Digital, however, downplayed potential concerns. They argued that Johnson has already successfully ushered through most critical legislation coming in the near future, save for the reauthorization of the Federal Aviation Administration (FAA), the deadline for which is May 10.

‘Absent the FAA reauthorization, which will pass later this year, Speaker Johnson has done the job entrusted to him,’ the House GOP aide argued. 

‘While his majority may shrink with another GOP resignation, he’s already won the tough legislative battles. Any dysfunction moving forward falls squarely on those who refuse to govern and prefer to complain.’

The aide was referencing members of the ultra-conservative House Freedom Caucus and their allies, who have wielded outsized influence over the House GOP’s thin majority by voting in small blocs to kill or block Republican leaders’ legislation in protest of their handling of critical matters like government spending and foreign aid.

The group has already signaled that they’re putting up a fight over another coming legislative battle — funding the reconstruction of the Francis Scott Key Bridge in Baltimore. 

The Freedom Caucus put out a statement earlier this month demanding that any funding allocated by Congress be offset by spending cuts elsewhere, and that the funds would solely focus on bridge reconstruction.

It’s not immediately clear when a funding bill could take shape or how much leverage conservatives have, given the strong bipartisan support it’s expected to receive. But GOP rebels are expected to give Johnson a hard time if he tries to pass it through traditional mechanisms that rely solely on party-line votes.