Ms. Collins, who provided a key vote in support of President Obama’s economic stimulus plan and backed the Wall Street regulatory overhaul, said Mr. Castle’s defeat had come as a shock. She attributed the rise of the conservatives to a backlash against Obama administration policies.

“It is stunning that he could be defeated in a primary, and it is very troubling to me,” Ms. Collins said in the recent interview. “I believe it is a reaction of the overreaching of the Obama administration and people’s fear and worry about the economy.”

But some Maine Republicans now say Ms. Snowe should be replaced with a more conservative candidate when she is up for re-election in 2012.