WASHINGTON — By definition, the majority leader of the House has the majority of incumbents to protect in an election. So it came as something of a shock when House Republicans learned that a political action committee affiliated with Representative Eric Cantor of Virginia, who currently holds that leadership slot, had donated $25,000 to a group devoted solely to taking out incumbents.

Mr. Cantor has been on the defensive ever since.

The story began when $25,000 from the Every Republican Is Crucial Political Action Committee was given to the anti-incumbent “super PAC” Campaign for Primary Accountability. Leaders of the group said they shrugged when they got the money and threw it into their $2 million anti-incumbent pot. They did not realize that Every Republican Is Crucial stands for something: Eric.

Mr. Cantor’s aides see it differently. That money, they said, was earmarked for the anti-incumbent group’s effort to defeat Representative Don Manzullo, a 10-term Illinois Republican who, because of redistricting, had found himself running in a primary against the freshman Representative Adam Kinzinger, a favorite of Mr. Cantor’s.

Ray Allen, a Cantor political consultant, said that in mid-March, Representative Aaron Schock, Republican of Illinois and a Kinzinger ally, asked Mr. Cantor if he would match his own donation to the Campaign for Primary Accountability, described only as an organization supporting Mr. Kinzinger.