This situation reminds me of the Ultumatum game.

Say there are three candidates: Alice, Bob and Chandrakant. Everybody knows Chandrakant doesn't have a chance. Between Alice and Bob, you prefer Alice, but would like her to support the ban on transgenic oysters. So, along with a small vocal minority, you threatedn to vote for Chandrakant if she doesn't support the ban.

This plays out like this:

Round 1, Alice decides whether or not to add the ban on transgenic oysters to her platform.

Round 2, you decide whether to vote for her or Chandrakant.

Alice's decision in round 1 depends on how seriously she takes your threat to vote for Chandrakant in round 2. If you're the kind of person who is known for "not wanting to throw his vote away", then you'll end up still voting for her and eating transgenic oysters. If you (and people like you) are known for being stubborn as a mule, she may change her platform.

(note that none of this requires that you actually prefer Chandrakant to Alice - voting for a third-party guy can also be strategic voting!)