Representative Mike Gallagher of Wisconsin, once seen as a rising star, made the announcement just days after voting against impeaching the homeland security secretary.
Representative Mike Gallagher, Republican of Wisconsin, announced on Saturday that he would not run for re-election, just days after breaking with his party to cast a decisive vote against impeachment charges for Alejandro N. Mayorkas, the homeland security secretary.
Mr. Gallagher, who is in his fourth congressional term, is joining dozens of other lawmakers who have decided to call it quits. But the timing of his decision was striking nonetheless, coming on the heels of his impeachment vote — which had already earned him a primary challenger — and his relative youth, compared with others who are planning to retire from Congress.
“Electoral politics was never supposed to be a career and, trust me, Congress is no place to grow old,” Mr. Gallagher, 39, said in a statement, adding that he had made the decision not to run “with a heavy heart.”
Mr. Gallagher, a Marine Corps veteran and a former congressional staffer, was an influential voice in the House when it came to matters of national security and the military. He was particularly outspoken about the wars in Afghanistan and Ukraine, as well as cybersecurity, having co-chaired an intergovernmental commission on the issue early in his congressional career.
Last year, when House Speaker Kevin McCarthy selected him to lead a new committee tasked with investigating threats posed by the Chinese Communist Party, he was the youngest Republican wielding a panel chairman’s gavel.
Mr. Gallagher also caught the eye of Senate Republican recruiters, who attempted last year to convince him to run against Senator Tammy Baldwin, Democrat of Wisconsin. But Mr. Gallagher decided against that bid, announcing at the time that he would seek re-election to the House.
His standing in the G.O.P. appeared to have shifted earlier this week, however, after he became the third House Republican to refuse to back the impeachment effort against Mr. Mayorkas. The charges, of refusing to uphold the law and breaching the public trust, were widely dismissed by legal experts as not meeting the constitutional threshold of high crimes and misdemeanors.
The effort to impeach Mr. Mayorkas failed by just one vote.
“The proponents of impeachment failed to make the argument as to how his stunning incompetence meets the impeachment threshold,” Mr. Gallagher said in a statement this week defending his decision, arguing that impeaching Mr. Mayorkas would “set a dangerous new precedent that will be weaponized against future Republican administrations.”
The House is expected to try to impeach Mr. Mayorkas again next week, once Representative Steve Scalise of Louisiana, the House’s No. 2 Republican who has been absent while undergoing treatment for blood cancer, returns to Washington.
Mr. Gallagher did not say precisely what he planned to do next, though he indicated that his next role would also be in the national security space.
“Though my title may change, my mission will always be the same,” he said in a statement. “Deter America’s enemies and defend the Constitution.”