House Republicans are panicking about their shrinking majority after Rep. Ken Buck (R-Colo.) said he will resign at the end of next week, stunning GOP leadership.

Why it matters: Republicans' razor-thin margins have created headaches throughout the 118th Congress — empowering hardliners, causing embarrassing defeats on key votes, and even contributing to the ouster of House Speaker Kevin McCarthy (R-Calif.).

The intrigue: Members of Republican leadership said they were blindsided by Buck's sudden announcement.

• "I was surprised by Ken's announcement — I look forward to talking with him about that," House Speaker Mike Johnson (R-La.) told reporters.
• House Majority Leader Steve Scalise (R-La.) said: "I haven't gotten to talk to him. He wasn't on the floor but I'm curious to see why he's leaving early."

By the numbers: Republicans will have 218 seats to Democrats' 213 after Buck leaves, meaning the GOP can lose just two of their members on any given party-line vote.

• The resignations of McCarthy and Rep. Bill Johnson (R-Ohio) — as well as the expulsion of Rep. George Santos (R-N.Y.) and the loss of his seat to Democrats — have led Republicans to this point.
• A special election in April to replace Rep. Brian Higgins (D-N.Y.) will likely net Democrats another seat.
• Republicans are then poised to win the May and June elections to replace McCarthy and Johnson, respectively. Colorado Gov. Jared Polis said he intends to set the special election to fill Buck's seat for June 25.

What they're saying: "It lowers the margin and that creates an obvious challenge for leadership," Rep. Steve Womack (R-Ark.) told Axios of Buck's resignation.

• "I am concerned about the majority," said Rep. Mike Kelly (R-Pa.), adding, "I just wish the rest of our party was."
• House Rules Committee Chair Tom Cole (R-Okla.) said of Buck's decision: "Obviously, it makes the numbers that much tougher."

Reality check: House Republicans have dispensed with many party-line votes out of fear of right-wing defections.

• Major bills are now routinely being passed through a process known as "suspension," which requires a two-thirds majority for votes to succeed.

Between the lines: What Buck's resignation means in practice is that "if it can go wrong, it probably will go wrong — at the worst time," Womack explained.

• On any given vote, he said, Republicans could be hobbled by "absences, through travel or illness or family hardship or whatever."

The other side: Some GOP lawmakers argued that Buck's resignation won't have a concrete impact on the day-to-day workings of the House.

• "We already have a close majority, so it just requires us to be just as vigilant as always," said Rep. Ben Cline (R-Va.).

The backdrop: Buck has evolved in recent years from a right-wing bomb-thrower to the preeminent Republican critic of efforts to impeach President Biden and Homeland Security Secretary Alejandro Mayorkas.

• He was one of three Republicans who voted against impeaching Mayorkas, much to the frustration of his GOP colleagues.
• "We're down to the point every vote counts. Ken is not always a predictable vote, he's a very independent vote, but ... I hate to lose him," said Cole.

Zoom out: Republicans are also worried about what Buck's resignation says more broadly about Congress as an institution at a time when a historic number of lawmakers in both parties are planning to retire.

• "Congress is like life, you have good years and you have bad years, and it's been a difficult Congress, no question about it," said Cole.
• Womack said he worries "there could be other members out here who have decided not to seek re-election that might choose to end their careers early because of other opportunities."

The bottom line: Asked whether he's facing heat from his colleagues, Buck told Axios: "I think it's the next three people that leave that they're going to be worried about."