Ambinder argues that the tea party folks are mostly right-leaning independents and that this is a least as much a problem for the GOP as the Democrats. Ambinder argues that these voters are unlikely to vote for a Democrat in any case so that if they are unsatisfied, they will either back splinter candidates or stay home. So if you count the tea partiers as already Republican votes, the best that the GOP can hope is to not lose (if the tea partiers show up and vote Republican) or just plain lose (if they stay home or vote for splinter candidates.
This makes several, almost certainly alse assumptions. It assumes that the tea party participants and their sympathizers were already in the Republican bag. National Review did a survey of tea party sympathizers and found that 68% of them supported had McCain. Thats alot, but that 32% shows room for growth over 2008 if those voters an be mobilized. That would qualify as a potential gain for the GOP I think.
Ambinder also ignores how the upsurge of tea party activism has brought thousands of Americans into a form of spontaneous(ish), decentralized, right-of-center political activism and creates the potential for expansion of right-of-center GOTV, fundraising, candidate recruitment and other forms of political involvement beyond voting. The tea parties are much to be preferred to the combination of apathy and despair that greeted that McCain campaign - Palin aside.
There are ways that the tea parties could lead the Republicans into error, but not in the ways that Ambinder describes. Where the Republicans nominate a credible candidate who is not an outright liberal, there is no evidence that the tea partiers will cause trouble. They might even have inspired some people to vote, donate or volunteer. Where Republicans nominate a Dede Scozzafava, they deserve what they get. My worry is that Republicans will confuse the voice of the tea partiers with the voice of the median American voter, who, when he or she looks back on the last eight years of Republican presidential administration, might not look at increased domestic spending as the biggest or worst error of Republican-led government.