Rep. Mike Gallagher to leave Congress next month, shrinking GOP's narrow majority

Republican Rep. Mike Gallagher, who had already announced he would not be seeking reelection, announced Friday that he's resigning from Congress next month -- shrinking House Republicans' already razor-thin majority even more.

The Wisconsin Republican said his last day will be April 19. His departure will give House Republicans a one-vote majority with 217 Republicans and 213 Democrats.

"I will forever be proud of the work I did on the Armed Services and Intelligence Committees, chairing the Cyberspace Solarium Commission, and chairing the Select Committee on the Chinese Communist Party. It has truly been an honor to serve in the House of Representatives," Gallagher said in a statement.

As chair of the House Select Committee on China, Gallagher and was the driving force behind recent House-passed legislation that would ban the popular video app TikTok unless it parts ways with its Chinese-owned parent company ByteDance.

Gallagher had previously announced he would not seek reelection shortly after he cast a vote to doom the first attempt to impeach Homeland Security Secretary Alejandro Mayorkas.

Gallagher did not provide a reason for why he's resigning early -- but said he made the decision after "conversations with my family."

"Four terms serving Northeast Wisconsin in Congress has been the honor of a lifetime and strengthened my conviction that America is the greatest country in the history of the world," Gallagher said in the statement.

There will not be a separate special election to fill Gallagher's seat, according to Wisconsin state law. Rather, the seat will be filled via the regular state primary and general election.

Wisconsin state law says, "A vacancy in the office of U.S. senator or representative in congress occurring prior to the 2nd Tuesday in April in the year of the general election shall be filled at a special primary and election. A vacancy in that office occurring between the 2nd Tuesday in April and the 2nd Tuesday in May in the year of the general election shall be filled at the partisan primary and general election."

Gallagher will be the fifth House Republican to resign early from the 118th Congress rather than serve out the term. Only one Democrat has left early (Rep. David Cicilline, who represented Rhode Island.)

Rep. Ken Buck, R-Colo., who had also said he would resign early after stating he wouldn't seek reelection, cast his final vote as a member of Congress Friday. His departure leaves House Republicans with a two-vote majority -- 218 Republicans compared to 213 Democrats.

ABC News' Oren Oppenheim contributed to this report.