Geoffrey Kabaservice is the author of Rule and Ruin: The Downfall of Moderation and the Destruction of the Republican Party, from Eisenhower to the Tea Party.

In 2010, John Danforth, a former Republican Senator from Missouri, was asked about the possibility of a GOP primary challenge to Indiana Senator Richard Lugar. Danforth pointed out that Lugar was a six-term Senator, one of the Senate’s most respected members, and its leading authority on foreign policy. He warned that “If Dick Lugar … is seriously challenged by anybody in the Republican Party, we have gone so far overboard that we are beyond redemption.”

Many commentators will draw precisely that message from Lugar’s defeat Tuesday night by his Tea Party-aligned challenger Richard Mourdock. Lugar was one of the few remaining Republican senators who might be described as moderate, and his loss weakens the already frail forces of bipartisanship, compromise, and comity on Capitol Hill. But it has been a long time since Lugar openly identified as a moderate Republican, and other factors besides intra-party factional warfare may have been principally responsible for his political demise.