So how does the Tea Party movement fit into this? As Mark Blumenthal notes, citing a CNN poll,
Remember the 52 percent of Tea Party activists who initially identify as independent? It turns out that virtually all of them lean Republican. According to CNN, 88 percent of the activists identify or lean Republican, 6 percent identify or lean Democratic and only 5 percent fall into the pure independent category.
Remember that CNN pollster Holland reported that 87 percent of the Tea Party activists would vote Republican if there were no Tea Party-endorsed third-party candidate running? That makes perfect sense for a group that is 88 percent Republican.
The point is that the Tea Party movement is not nearly as exogenous as it seems. It is a movement of a certain type of Republican-leaning independent. And that's why, all other things being equal, Tea Partiers are likely to be as damaging to Republicans as they are to Democrats. But didn't I just suggest that they would vote for Republicans anyway? No -- I said that their voting history suggests that they've voted, past tense, for Republicans -- which means that, should they decide to form third party candidacies, they will also certainly draw votes from Republicans. There is a distinct ideological dimension to the Tea Partiers; they are "fed up" with both parties, but they are, quite plainly, more "fed up" that the Republican establishment does not seem to embrace their emotional valence. If they're satisfied, they'll vote Republican.
If they're disatisfied, they won't vote Republican. (They won't vote for Democrats in either case.) I would hazard a guess that Tea Partiers, though they exist in marginal districts, tend to be more numerous in heavily Republican districts. So perhaps we overstate the degree to which Tea Party enthusiasm, per se, translates into Republican political success in the midterms. There's no denying that Republicans generally are more enthusiastic and that this enthusiasm will help them relative to the Democrats, but whether the Tea Party contingent of the party ultimately helps or hurts Republicans depends a lot on factors we don't know just yet.