When Sharron Angle won the GOP Senate nomination to run against Harry Reid, the tea party movement rejoiced. Then the post-primary polls came in. Instead of the pre-primary sense that Reid would no longer be a senator as of January, it looked as if he'd hold his own against that particular candidate. I think the polls are still bearing that out, although that sort of thing can change in a few months. The tea party movement endorsed someone who may well be incapable of beating Harry Reid, whose popularity is very low right now, even in a very Republican year.
The Arizona primary produced several similar situations. Consider Scott Elliott's preview of four House GOP primary races on Monday (the primaries were held on Tuesday).
AZ-1: "Ann Kilpatrick is one of the most vulnerable Democrats in the country" but "If Dr. Gosar wins, however, there will be some from the "McCain/Establishment" side that vote against him in November out of anger. In that case, we'll have to see whether their votes will be enough to cost him the victory." Gosar won.
AZ-3: "Ben Quayle has never had any major support from any Arizona politicians. He has sold himself as the Tea Party candidate. A strong front-runner at one point, Qualye has fallen sharply after several major debacles (i.e. borrowing kids for paid media photos, admitting to writing posts for a female-bashing website) .... should Quayle win this race, his baggage will make it hard for him to keep conservative Democrat and father-of-five, John Hulburd from taking this one for the Democrats." Quayle won.
AZ-5: "David Schweikert has ran for office twice before and lost, and Susan Bitters Smith was the 2008 nominee that Mitchell easily defeated.... If either of these candidates wins the primary, it's not likely they can unseat Mitchell." Schweikert won.
AZ-8: "Kelly was an early favorite, and attempted to define Paton as the "establishment" Republican. However, Paton's record as a legislator proved him to be a strong conservative, who continually takes on the governor and the leadership of both parties. The race between Kelly and Paton is very close, and either could win the primary. However, while Paton almost certainly will defeat embattled Democrat incumbent Gabrielle Giffords in November, Kelly may have a tougher time. He'll have to prove he's about more than just border security." Kelly won.
Now not all of these are tea party victories over establishment or moderate opponents (and the Florida primaries the same day had several GOP winners who would be easier winners in November than their opponents). But all of them were the choice Scott predicted would make it harder for Republicans to win the seat, and I think the tea party movement played a big role in the win of at least three of the four. It's clear that the tea party movement, while helping Republicans regain some ground against Democrats, may also prevent them from gaining as much as they otherwise might.