The electoral college guarantees a two party system in the general election. Any "third party" has to come through one of the two primaries. But honestly, in most cases if you can't win the primary of the party that more closely matches your beliefs, then you can win a general with those same voters plus the other party.
ETA: I would prefer a general election with a runoff as Louisiana does for governor.
This post was edited on 1/26 at 7:45 pm