Representative Kevin McCarthy of California abruptly withdrew on Thursday from the race to succeed Speaker John A. Boehner, blindsiding his House Republican colleagues and throwing their already tumultuous chamber into deeper chaos with no clear leader in sight just weeks before a series of high-stakes fiscal battles.

As lawmakers ate barbecue and sipped sodas during what was expected to be a pro forma vote to select Mr. McCarthy as their nominee, he did an about-face, saying that he had concluded he could not unite the increasingly fractious Republican majority.

“I am not that guy,” said Mr. McCarthy, with his wife and family by his side, according to members who were in the room. Moments later, Mr. Boehner, who learned of Mr. McCarthy’s decision only minutes before he announced it, declared the vote postponed and the meeting adjourned even though there were two other candidates in the running, underscoring the weakness of the field.

Some Republicans, including Mr. Boehner and Mr. McCarthy, are pressing Representative Paul D. Ryan of Wisconsin, the party’s nominee for vice president in 2012, to step up. Mr. Ryan, however, has repeatedly said he does not want the job, a point he reiterated Thursday even before his colleagues had left the meeting.