What impact have third parties had on the United States?

Under the rules of the Commission on Presidential Debates, presidential candidates must earn the support of at least 15 percent of voters in national polls in order to join the televised debates; recent reports suggest that Libertarian Party candidate Gary Johnson may be getting close. With less than two months to go until the first debate, he is hitting between 8 and 11 percent in various national polls - still well behind the nominees of the Democratic and Republican parties, but enough to make an impact on the outcome.

Barbara Perry, the director of presidential studies at the University of Virginia's Miller Center and co-chair of the center's Presidential Oral History program, recently discussed the impact third parties have had over the years and how they might affect the 2020 election.