Third parties are a bad idea for American presidential elections, and here's why:
The Electoral College + the Twelfth Amendment.
The problem: The EC requires a majority, not just a plurality, of 270 EVs. If you bring in a competitive third party, the EVs get split three ways. If no one hits 270 EVs, the presidential election goes to the House. The House also requires a majority, not just a plurality, of state delegations. If you bring in a competitive third party into the house, the state delegations get split three ways. If no one hits 26 state delegations, the House just has to keep voting, and the Speaker of the House serves as President in the meantime. Structurally, as it is, the Republicans have a major advantage in state delegations, such that if no one reaches 270 EVs, the Republicans will pick the president. This is either a bug or a feature depending on perspective, but either way it's not friendly to a third party.
The lack of solution: Ranked choice voting does not solve this. That just makes it more likely that third parties will screw up presidential elections. The NPVIC does not solve this, because our current Supreme Court would never let it take effect. A constitutional amendment will not overturn it because one of the two major parties has a structural reason to leave it in place. The Electoral College + the Twelfth Amendment are here to stay. There is no way around it.
The Electoral College + the Twelfth Amendment.
The problem: The EC requires a majority, not just a plurality, of 270 EVs. If you bring in a competitive third party, the EVs get split three ways. If no one hits 270 EVs, the presidential election goes to the House. The House also requires a majority, not just a plurality, of state delegations. If you bring in a competitive third party into the house, the state delegations get split three ways. If no one hits 26 state delegations, the House just has to keep voting, and the Speaker of the House serves as President in the meantime. Structurally, as it is, the Republicans have a major advantage in state delegations, such that if no one reaches 270 EVs, the Republicans will pick the president. This is either a bug or a feature depending on perspective, but either way it's not friendly to a third party.
The lack of solution: Ranked choice voting does not solve this. That just makes it more likely that third parties will screw up presidential elections. The NPVIC does not solve this, because our current Supreme Court would never let it take effect. A constitutional amendment will not overturn it because one of the two major parties has a structural reason to leave it in place. The Electoral College + the Twelfth Amendment are here to stay. There is no way around it.