WASHINGTON  The few remaining Republican centrists in the Senate were eagerly awaiting the arrival of Michael N. Castle of Delaware, a longtime and reliable moderate voice who could provide some counterbalance to the wave of conservatives poised to enter Congress and the steadily rightward shift of party leaders.

But Mr. Castle was defeated in his party primary on Tuesday by Christine O’Donnell, a Tea Party insurgent. And while the conservative wing rejoiced, the surprise outcome raised serious questions about the future place in the party of lawmakers like Senators Olympia J. Snowe and Susan Collins of Maine and other Republicans in the Senate and House who are not lock-step conservatives.

Senator Jim DeMint, the South Carolina Republican who has seen his stature rise through his support of conservative candidates, made it clear in the aftermath of the Delaware upset that he would prefer losing a seat to Democrats than having Republican colleagues who stray from the conservative line and erode party unity and image by voting for policies supported by the Obama administration.

The ascendancy of the right is forcing even some of the most loyal Republicans, like Senator Lamar Alexander of Tennessee, the No. 3 Senate Republican, to defend their conservative bona fides. And it seems to be dashing the hopes among moderates that the prospect of winning a majority in the House, and a pursuit of independent voters, would push Republican leaders to the middle.