The move towards liberalism will be strengthened by a McCain defeat, not negated. The Republicans will see that the only way to win is by moving towards the center (which then moves left), not towards the right. That's where we got "compassionate conservatism" (the Republicans having believed in '00 that the only way to win was to play that game). Pulling conservatives out of the party only facilitates this movement, especially when the party sees a conservative on the ticket but it loses anyway.
Think of it like this: if McCain wins this year, he is unlikely to run for a second term. There's no question that Palin won't be more heavily involved than any other VP in recent memory (perhaps ever) because 1) she will represent the "new" face of the Republicans, 2) she will be a stick in the eye of the Dems, 3) even the limited exposure she's received indicates she's got more of a speaking ability than McCain does, and 4) she doesn't seem the type to stay home and bake cookies for the White House. So, you can either look to Obama up for 8 years, or set Palin up for her first full term.