The US electoral system is set up so that building up enough votes for a third party to win means losing multiple elections to the opposition. Trying to build up, say, a Libertarian vote means losing elections to the Democrats, just as trying to build up a Green vote means losing elections to the Republicans. It's hard to hold a third party vote together long enough to get anywhere in the US system. The only time it was ever been done is when the Whigs split over slavery, when the Republicans became the second party, replacing the Whigs, and then the only reason the Republicans won the Presidency was because the Democrats split over slavery again.
Now, if the instant runoff or range voting was used, and then proportional representation, matters would be different—a vote for a third party would have less of a chance to put your opposition in power. Over to:
- The Center for Voting and Democracy (http://www.fairvote.org/), which advocates the instant runoff, proportional representation, and several other voting reforms.
- The Center for Range Voting (http://rangevoting.org/).
- Common Cause (http://www.commoncause.org), which has been working on money and media issues since 1970s.
- Professor Douglas J. Amy's proportional representation archive (https://www.mtholyoke.edu/acad/polit/damy/prlib.htm).