This Associated Press article goes into all the reasons Sen. Richard Lugar is being challenged from the right in his party and why some see him and Orrin Hatch, the Tea Party's other main target this year, as "old bulls out of touch with today's conservatives." I think this gets to the heart of it:
Monica Boyer, a tea party activist, said she had always voted for Lugar because he was a Republican. The tipping point, she said, was when Lugar voted to confirm President Obama's Supreme Court nominees, Sonia Sotomayor and Elena Kagan. That was a "hard wake-up call," she said, that spurred tea partyers to dig into Lugar's voting record.
I remember Lugar justifying his support for Sotomayor and Kagan as reflecting historical Senate practice: If there isn't something radically wrong with an appointment, the president has the right to appoint whomever he wants, and we shouldn't oppose them on philosophical grounds. And he was right, that is the way things were done. But going along with the "way things have been done" is exactly the Tea Party's complaint. What then was seen as bipartisan statesmanship is now seen as accommodation that allowed the government to mutate into something monstrous.
Lugar's vote on the justices show him trying to remain a gentlman in a place that doesn't play by gentlemen's rules anymore. There is a partisan divide these days that amounts to a war. We can lament the divide, or blame one side or the other for it depending on our own predilections, but the fact is it's there. And if the other side isn't playing by the old rules and Lugar still tries to, it amounts to unilateral disarmament.