This is a good argument. +1, but I still have reservations.
There is a bit of a difference, though. When you supported Obama, I assume it was to elect someone that you believed would do a good job as Chief Executive, overall. It wasn't in order to pass one piece of self-serving legislation. The fact that you weren't acting purely in your own interest, and that the interest was broader than one single issue, makes it somewhat different. But different enough to overcome your objection? I'm not sure.
Many people make their voting/donation choices based on a single self-serving issue. Is it wrong for sexually active people who don't want children to vote/donate based on abortion, or for gays to vote/donate based solely on gay marriage?
There is a bit of a difference, though. When you supported Obama, I assume it was to elect someone that you believed would do a good job as Chief Executive, overall. It wasn't in order to pass one piece of self-serving legislation. The fact that you weren't acting purely in your own interest, and that the interest was broader than one single issue, makes it somewhat different. But different enough to overcome your objection? I'm not sure.