I've never known a US presidential election to be upset by a third party candidate. There have been realigning elections in which a third party gains more ground during campaigning and overtakes an established party (according to Wikipedia, this happened in 1856 when Republicans replaced the Whig party). This isn't the same as that little fraction of voters who pick a candidate that isn't in the top two at the primary election. A lot of voters just pick whatever they consider to be the lesser evil instead of their actual favorite candidate. Do those third party voters make good use of their votes?