Any party whose only success was swinging California away from one candidate, for example, would see its platform gobbled up by the other parties. Sometimes you have to support the foot in the door. But if you believe that Bush's position isn't strong enough, you still want to vote for someone who holds a stronger position. Marijuana prohibition is a good example of what happens when a vote is seen as unswingable. Every state that has taken up the issue of medical marijuana as an initiative has voted, usually overwhelmingly, in favor of letting doctors prescribe it.
Politicians continue to steadfastly oppose it.
Even Bush's support is only a states' rights issue. In my opinion, because the medical marijuana vote is seen as a Democrat, or 'liberal', vote. Medical marijuana activists are seen as afraid to vote Republican. Democrats don't need to court their vote, and Republicans can't gain anything by courting it. Unless the medical marijuana community can change that perception, their issue will continue to lose in the legislature.