Third parties or movements can affect the platforms of the two major parties, even if the third parties don't themselves seem strong. When the shouting of those in the third parties gets loud enough, they get attention. In the case of the Tea Party movement, Republican candidates are tripping over each other to pander to them.
If the Republicans do enough right to satisfy most in the TP movement, there won't be a need for them to field their own candidates.
On the other hand, if the Republicans return to business as usual, it's possible that the Tea Party movement could die out due to disillusionment. Many, if not most, of the people in the TP movement are active for the first time politically, and so haven't been through the wringer. A couple of kicks to the head may be all that's needed to cause people to lose hope.
I know that sounds cynical, but it's the game the Republicans played with gun rights supporters in Wisconsin for years. They'd throw a bone once in a while and actually deliver on a promise. Then they'd use the carrot again to get the gun rights people to give money and time, only to fail to deliver in the next session. "We tried our hardest," they'd say, when in fact they intentionally failed. If they delivered on every promise, there wouldn't be enough carrots left.
Those kinds of games burn out newcomers fast, but the politicians know there's more newcomers to be had. With a movement as large as the Tea Party, there may not be a sufficiently large supply of new idealists should the current crop be screwed by the Republicans.
