Former Speaker Kevin McCarthy’s announcement that he would leave Congress came as little surprise to his closest colleagues, but his decision to do so a year before the end of his term poses challenges for his party. It will shrink Republicans’ already razor-thin majority in the House as they face a number of issues in the coming months that will require near-unanimous party support.

The departure of Mr. McCarthy, who was his party’s strongest fund-raiser in the House and spent two election cycles helping to build the Republican majority, also could put a dent in the G.O.P.’s ability to rake in campaign cash, although he has said he wants to continue to play a role in politics.

Here’s how Mr. McCarthy’s departure could affect the House and his party.

Republicans started the year acknowledging that one of their biggest challenges would be keeping their party unified as their midterm victories delivered a tiny majority. They had 222 members while Democrats had 213, leaving little room for defectors and making it easier for a small number of disgruntled Republicans to influence policy and vote outcomes.

They could afford to lose no more than four votes on any bill if all Democrats showed up and voted against them. Any more than that would doom G.O.P. legislation.

With the expulsion last week of former Representative George Santos of New York, Republicans now have only 221 members, meaning their four-vote margin has shrunk to three. Any more defections than that would result in a 217-to-217 tie or give the Democratic side more votes than the Republican one.

With Mr. McCarthy gone, Republicans will enter the new year with 220 votes, leaving the same margin since they could still lose three votes and be ahead of Democrats, 217 to 216.

A special election for Mr. Santos’s seat is set for Feb. 13, and Democrats hope to recapture the politically competitive district, which President Biden won in 2020. That would further erode the Republicans’ edge.

Gov. Gavin Newsom of California will have 14 days after Mr. McCarthy’s final day to call a special election, which must take place about four months later. The Bakersfield-anchored district is solidly Republican, meaning that a G.O.P. candidate is likely to win the race to serve out the remainder of his term. But that won’t happen before mid-January, when lawmakers face the first of two deadlines for funding the government.

Speaker Mike Johnson, Republican of Louisiana, has struggled to push critical legislation through the House, and a slimmer majority would probably empower the rebellious hard-right wing of his party to double down on its policy demands ahead of the deadlines, the second of which is in early February.

The smaller majority could also affect the fight over an emergency national security spending bill to fund the war in Ukraine, along with help for Israel in its war against Hamas and border security funding.

On Wednesday, Republicans blocked the measure in the Senate. The bill would face an uphill battle in the House, where Republican support for Ukraine’s war effort is dwindling.

For years, Mr. McCarthy has traveled to hundreds of districts across the country, bringing in millions of dollars in campaign cash for candidates and helping Republicans win control of the House in 2022. He has said he planned to remain engaged in Republican politics.

“I will continue to recruit our country’s best and brightest to run for elected office,” Mr. McCarthy said in announcing his plans to leave the House in The Wall Street Journal. “The Republican Party is expanding every day, and I am committed to lending my experience to support the next generation of leaders.”

During his time as speaker, Mr. McCarthy brought in $78 million for his colleagues’ re-election efforts, more than 100 times the amount of money Mr. Johnson had collected before becoming speaker.

His support of new candidates will be aided by a campaign account with more than $10 million at his disposal. Even after leaving office, Mr. McCarthy can use the campaign funds to establish a political action committee or directly support other campaigns. He has signaled that he would like to play a substantial role, and many lawmakers and aides believe he may intervene in party primaries to target the far-right Republicans who led the push to oust him from the speakership.

More than three dozen incumbents from both parties in both chambers have said they will not seek re-election. If even a handful more House Republicans leave in the coming months, it could wipe away their majority before a single vote is cast in the 2024 election. Another Republican, Representative Bill Johnson of Ohio, has announced that he will leave Congress in several months to become the president of Youngstown State University, though he has not said precisely when.

Representative Marjorie Taylor Greene, Republican of Georgia, and one of Mr. McCarthy’s strongest allies, expressed her frustration over the eroding majority in a post on social media, saying, “Hopefully no one dies.”