More than 80 percent of Republicans and Democrats are going to go with their current choices, even if someone somewhat appealing pops in. If I weren’t being so conservative, I’d say it’s closer to 90 percent. John Anderson dropped out of the Republican primary and ran as a general liberal in 1980. He polled well, at first. But eventually, voter behavior took over, Ronald Reagan successfully sold himself not as a crazy kook but a legitimate candidate, and in the end Anderson won no states outright. His strongest performances swung states to Reagan by eating away at Jimmy Carter’s liberal voter bloc in Massachusetts, New York, etc.
Third parties play spoiler every time. The last with a candidate who wanted to play kingmaker was George Wallace in 1968.
What voter is really going to cast his ballot for someone he knows cannot be president, just to block another person from winning? They have to believe the option they are selecting has a chance, or they will naturally drift back to option A or B. Sure, people who like to think up hypotheticals all day and fantasize about their single vote deciding a race may be persuaded here. But this isn’t how normal people think. People in the end vote for or against a person, not “Screw it all, I’ll vote Z.” If they are that dissatisfied, they aren’t showing up.