Gallagher, who chairs the House of Representatives Select Committee on the Chinese Communist Party, had already said he would not be running for re-election in November. But his decision to leave Congress early means the seat will be vacant for a time before the next election.
Spokespeople for Gallagher did not immediately respond to requests for comment.
The Republicans currently hold a 219-213 House majority. However, Republican Representative Ken Buck is due to step down on Friday, narrowing the majority to four seats.
When Gallagher steps down, that will narrow the majority further to 217-213, meaning that Speaker Mike Johnson can only afford to lose one vote in order to pass legislation if Democrats remain united in opposition.
The fractious Republican majority has proven difficult to manage, and Johnson has repeatedly relied on Democrats to pass critical measures, like a government funding bill approved by the House on Friday. Anger over that led Republican hardliner Representative Marjorie Taylor Greene to threaten to call a vote to oust Johnson as speaker, six months after the party booted its prior leader, Kevin McCarthy.
Gallagher communicated to staff he would step down on April 19, said the sources, who requested anonymity to discuss private matters.
It was not immediately clear why he planned to step down early.
(Reporting by Gram Slattery; Editing by Scott Malone)
By Gram Slattery and Michael Martina
