Voting against the incumbent Republican or Democrat by voting for the Democratic or Republican challenger is nearly futile. It is still voting for the monopoly of the incumbent parties and gives them little incentive to change policies or stop stifling "entry level" candidates with fresh ideas.
The major parties are not going to improve significantly until they get a "vote of no confidence" for their present conduct. If their party candidates get enough votes so that no candidate for office receives a majority, the major parties will have to try to reduce voter dissatisfaction instead of ignoring or exploiting it.
I'll be voting Libertarian every chance I get and Peace and Freedom or American Independent whenever those are the only third-party choices.
D. R. KELLER