It is quite well known by now that there is two-party dominance in the United States because of the first-past-the-post (FPTP) -- or plurality -- electoral system. Third parties can have an unintended “spoiler effect” on contests by stealing votes from the ideologically similar -- but more viable -- candidate.
For instance, in Virginia’s 2013 gubernatorial election, Libertarian candidate Robert Sarvis siphoned nearly 150,000 votes from Republican Ken Cuccinelli’s total, which was enough to give Democrat Terry McAuliffe a 55,000 vote edge, and the governor’s seat.