Even after the midterm rout that will remove many long-serving members from Congress, the idea that Mr. Lugar would be vulnerable to a primary challenge is a chilling notion to many Republicans, a symbol of symbolism gone too far.

“If Dick Lugar,” said John C. Danforth, a former Republican senator from Missouri, “having served five terms in the U.S. Senate and being the most respected person in the Senate and the leading authority on foreign policy, is seriously challenged by anybody in the Republican Party, we have gone so far overboard that we are beyond redemption.”…

Mr. Lugar’s recent breaks with his party have stirred the attention of Indiana Tea Party groups, who have him in their sights. “Senator Lugar has been an upstanding citizen representing us in D. C.,” said Diane Hubbard, a spokeswoman for the Indianapolis Tea Party. “But over the years, he has become more moderate in his voting.”

Removing him “will be a difficult challenge,” Ms. Hubbard conceded. “But we do believe it’s doable, and we think the climate is right for it and we believe it is a must.”