There are always strings attached. Let's say you give money to support a politician who opposes SOPA. When you give the money you are perfectly free to say "I'm contributing to your campaign because you are against SOPA".

Would it be extortion for you to tell a politician "If you reverse your stance and later support SOPA, I can no longer in good conscience continue to contribute to your political campaign"?

Clearly in a legal sense? Honest question.