If the mushy middle Bush stands to gain outnumbers the fundamentalist right Bush stands to lose, it's simple mathematics. I think that's the case right now. Instead of gaining influence, you may be marginalizing your voice.
I would submit that being willing to support someone simply because of an "R" behind their name marginalizes people far more than a demonstrated willingness to not vote for unacceptable candidates.
If you're in their pocket, what control can you really exert? They know they have you, and take you for granted.
You know how many conservatives get disgusted with the fact that 90% of the black population votes Democrat election after election, and how so many on the right are quick to point out that the Democrats "take them for granted"? Do you think the same can be said for many conservatives with respect to the Republican Party? I certainly do.