Delaware: There is no question that Christine O'Donnell was a terrible candidate. It isn't quite so clear that she was Tea Party candidate. The race was mostly ignored until a few weeks before the primary, when many national conservatives endorsed her to stop moderate Republican Congressman Mike Castle (ACU life 52%) from winning. O'Donnell was more a traditional conservative than a Tea Partier, but I won't split hairs here.
Castle was definitely a stronger candidate, but it is a myth that he was a shoo-in. The CNN exit poll showed that Castle would have lost to democrat Chris Coons 44-43. And that's without the democrats having spent millions attacking him, as they undoubtedly would have. Instead, the left spent a lot of time and effort attacking a Republican who was going to lose anyways. So even in what would seem to be the strongest case for the anti-Tea Party side, the facts don't support them.
Nevada: Sharon Angle won the Republican primary to face Harry Reid over Danny Tarkanian and Sue Lowden. She lost by 6% to Reid. Angle was a weak candidate, though not in the same category as O'Donnell. On the other hand, her primary opponents were hardly strong candidates either. Tarkanian has now lost four bids for office (state senate, SOS, US Senate, congress). Lowden was a one-term state senator whose campaign lost steam after derisible chicken-bartering comments. It is far from obvious that either of them would have outperformed Angle by 6%. I agree that Dean Heller or Jon Porter would have won, but they chose not to run.
Colorado: Ken Buck beat Jane Norton in the Republican primary before losing to democrat incumbent Ken Bennett. As far as I can tell, Buck was a decent candidate, but he was smeared as anti-woman for not prosecuting an alledged rapist due to lack of evidence. Perhaps Norton would have done better, but it is worth noting that she was elected Lieutenant Governor of Colorado on a ticket with incumbent Bill Owens, not on her own. There is no evidence that she was an electoral powerhouse.
Alaska: Joe Miller beat Lisa Murkowski in the Republican primary. Murkowski ran as a write-in candidate and won the general election. Murkowski was a stronger candidate, but from what I have heard, Miller still would have beaten the weak democrat nominee.
Missouri: Congressman Todd Akin beat Sarah Steelman and John Brunner in the Republican primary before losing to democrat incumbent Claire McCaskill. Akin was a bad candidate who destroyed his campaign with rape-related remarks. But he was not a Tea Party candidate; he found his strongest support from social conservatives. Steelman had some Tea Party and establishment support, and Brunner had some Tea Party and libertarian support.
Indiana: Richard Mourdock, state treasurer of Indiana, defeated incumbent senator Richard Lugar in the primary 61%-39% before losing to democrat congressman Joe Donnely. Mourdock was a Tea Party candidate who also had some local establishment support. He was twice elected state treasurer, winning 62% in 2010. I am not aware of any widespread claims that he was a weak candidate before the primary.
Lugar had long won by large margins, but he had not faced a serious challenge since first being elected to the senate, and was unopposed in 2004. He was out of touch with Indiana, had no residence in the state, and ignored clear signs that his campaign was in trouble until it was too late. He could have (I believe would have) experienced a similar collapse in the general election if he had coasted through the primary. Joe Donnely could have run to the right of Lugar on guns (Lugar had an NRA F rating), immigration, and possibly foreign policy, just as Mourdock had done.
Incredibly, there is not a single clear case of the Tea Party costing Republicans a US Senate seat. Possibly some of their primary opponents would have won, but it is easy to imagine fantasy candidates doing better than real-life candidates. Real candidates are imperfect and make mistakes.
Moreover, the "O'Donnell and Angle cost us the senate" argument is disingenuous because it cherry-picks evidence. It ignores the Tea party candidates who did win, and the establishment candidates who collapsed.
Tea Party candidates who won:
•Kentucky: Rand Paul was at least as strong as his primary opponent.
•Florida: Marco Rubio beat RINO turned independent Charlie Crist and became a national star.
•Pennsylvania: Pat Toomey pushed out Arlen Specter and narrowly won a dem-leaning state.
•Wisconsin: Ron Johnson found the right appeal to win a swing state.
•Utah: Mike Lee won the primary in a state to Republican would lose.
•Texas: Ted Cruz easily won the general after a tough primary.
Establishment candidates who collapsed/underperformed:
•Colorado governor: Scott McKinnis imploded in a plagiarism scandal.
•Kentucky governor: David McWilliams ran a terrible campaign.
•North Dakota senate: Rick Berg was establishment all the way, and lost a state other Republicans won easily.
•Wisconsin senate: Tommy Thompson ran a listless campaign and lost to leftist Tammy Baldwin.
•Montana senate: Denny Rehberg ran a weak campaign.
It would be just as disingenuous to say "Establishment candidates Berg and Thompson cost us the senate, so we should never support an establishment candidate again!" That would also be cherry-picking.
Beyond specific races, the Tea Party infused volunteers into the Republican party and provided a boost in many races where no Tea Party candidate was running. But I doubt this would have happened without Tea Party candidates on the ballot.
The lesson here is not that Republicans shouldn't nominate Tea Partiers, but that Tea Partiers should make an effort to find stronger candidates. Now that they have proven that they can beat incumbents and establishment favorites, that should be easier to do. Both Tea Partiers and social conservatives need candidates who are experienced at defending conservative beliefs to unfriendly audiences, not just "preaching to the choir". They found such candidates in Rand Paul, Mike Lee, Ted Cruz, Pat Toomey, and Marco Rubio, and they can find and elect more.