I would support a viable multi-party system, but the USA doesn't currently have one. If you vote third party, the fact of the matter is that in almost every single case you've just thrown away your vote as a meaninless protest vote that's equivalent to simply staying home and not voting at all. That vote you and a small minority of others don't cast for a Republican could have been sufficient to get the Democract past the post first, depending on the district.

Right now, it's a very simple proposition - for or against the Republicans. They themselves have forced that situation. If you try to play any other game, you're dividing their opposition and essentially helping them.

Altering the system to encourage more than two parties (which I think would be politically much healthier) is not a fight to fight in the current environment, in my opinion.