Lots of complaints on this blog and elsewhere about the decision of Indiana GOP primary voters to nominate Richard Mourdock instead of letting Richard Lugar continue to seek his seventh term. Of course, it is impossible to re-run history to see what would have happened, but I suggest that Lugar's re-election would have been far from a slam dunk.
First of all, Sen. Lugar was much more vulnerable than was widely realized. And many of the reasons for that vulnerability could have been exploited in a general election as well as in a primary. The Dems hadn't seriously challenged him for many years, so his popularity had not been recently tested.. And he had alienated lots of people besides the Tea Party by focusing heavily on national and international affairs with little attention to Indiana. A big part of Mourdock's appeal was the perception that Lugar no longer represented Indiana. One specific issue that Donnelly could have used to great effect was that Lugar hadn't even maintained a residence in indiana for over 30 years. There was a question whether Lugar was even eligible to represent Indiana, which I suspect would have received far more favorable press once it was Democrats instead of Tea Party activists raising it.
Second, Mourdock was a pretty strong candidate in his own right. Had Lugar stepped down, Mourdock would surely have been on the short list of candidates for the state GOP to recruit for the race. He was the state's top vote-getter in 2010, running a bit ahead of Sen. Coats. And he had a strong reputation for his grass roots support.
And remember that the primary ended up not being particularly close - Mourdock won 61-39. He didn't just have Tea Party support, but also had lots of veteran party activists behind him - it was GOP county chairmen who first persuaded him to run. And after the primary, all of Lugar's prominent supporters quickly rallied around Mourdock. In the primary itself, there was very little negativity towards Mourdock by anyone from Indiana besides the Lugar campaign itself. So Mourdock had as united a party behind him as Lugar would have had.
Certainly there were people who stayed home or voted for Donnelly who would have voted for Lugar. But I expect there were many others who came out because of Mourdock's grass roots efforts, or who would have voted Donnelly or Libertarian had Lugar been the nominee. It's hard to imagine Sen. Lugar losing a general election, but it was also hard to imagine him losing a primary by 22 points or Mourdock losing a general election here. Since those both happened, it is very possible that Donnelly would have beaten Lugar if he'd won the nomination. Also very possible that Lugar would have lost a primary to a weaker candidate if Mourdock hadn't run - State Sen. Mike Delph was rumored to be considering a run before Mourdock consolidated the anti-Lugar forces.
Finally, I'd point out that Mourdock was a team player with his advertising. The millions of dollars of ads against Donnelly mostly attacked the "Obama-Pelosi-Reid agenda." Since Donnelly responded by trying to distance himself from the national Dems, the attacks on the national Dems weren't answered. That helped Romney win easily here, and also aided our house candidates who added a seat to the GOP column.