WASHINGTON — Fearing that a seat crucial to winning a Senate majority could slip away, the national Republican establishment on Monday unleashed a furious campaign to drive Representative Todd Akin, the party’s newly selected nominee, out of the race against Missouri’s Democratic senator.

Amid an uproar over provocative comments on rape and abortion that Mr. Akin made in an interview broadcast on Sunday, the National Republican Senatorial Committee declared that it would withdraw financial and organizational support for Mr. Akin, including $5 million in advertising already reserved for the fall. In the interview, Mr. Akin said victims of “legitimate rape” rarely got pregnant.

Crossroads GPS, a Republican advocacy group that had already spent more than $5 million to weaken Senator Claire McCaskill of Missouri, considered the Senate’s most endangered incumbent, announced that it was withdrawing from the state.

At the same time, Republican candidates like Mitt Romney and Senator Scott P. Brown of Massachusetts either called for Mr. Akin to step aside or strongly indicated that he should. In a radio interview, the conservative host Sean Hannity pleaded with Mr. Akin to drop out. “Sometimes an election is bigger than one person,” he said.