Well you must do what you must do, but before you start patting yourself on the back for "voting your conscience", think of your conscience of the future.
If you take enough votes away from the much more conservative candidate and hand the people of this country a John Kerry or a Howard Dean and whomever either would appoint to the SCOTUS, you should feel shame for a whole generation or two.
You know a third party will not win and thus, you know who you help by playing this little game of taking all your marbles and going home.
You may well end up a worse enemy to the future of America than a proud liberal.