It's not about attacking a 3rd candidate - it's that this is a 2 party system. You can fucking hem an haw all you want - you can believe whatever the fuck you want. You can be the most leftist fucking tankie or evangelical rightwing nut job. It doesn't matter - be what you want, follow your ideals.
But when it comes to voting - you have 1 chance to make your vote count that's - during the primaries.
After that you tow the line and vote D or R - because that's the only way you will see the SLIGHTEST amount of change in the direction you want.