In that situation, the moral option is active resistance, not "neither." There is always the third option of active resistance. But that isn't what you're talking about. You're talking about throwing away a vote on a third party with no hope of winning and patting yourself on the back like you've accomplished something.
The choice between Republic and Democrat is not such a bad one that active resistance is morally preferable. But even if it was, voting for a third party is foolish and cowardly, and cannot be compared with true resistance to a corrupt government like those of Hitler or Stalin.