That's an interesting scenario. If they win, the Tea Party will try to take the credit. They'll clamor for more power. Will the GOP give it to them, or will they try to sideline them and continue trying to walk the middle path? With Romney, I'd bet on the latter. As every Tea Partier knows, Romney is not a true Right-winger.
This will only exacerbate the split in the party and may even lead to outright intra-party warfare, pretty much guaranteeing a one-term Romney presidency.
Anyway, that's the most likely scenario I see. Of course, what really happens depends on the economy, something that is only minimally affected by what US party is in power. If it improves greatly during the Romney presidency, something I find unlikely regardless of who wins, all rifts will be forgiven, at least temporarily. But if balancing the budget is to occur, then some hard choices are going to be made, and I do not expect the Tea Party to be rational about this. They do not have a history of being rational.