Reply #2: Yes, it has to be a majority of electoral votes.

[MineralMan]

Still, we haven't seen much of a threat from those third parties. They don't usually get any electoral votes, and I don't see that as a major threat in the future. Let's say, for example, that a third party on the left fielded a candidate. How many states do you suppose that candidate would win, or even get a substantial minority of votes? I'd think exactly zero. Third parties don't do well in Presidential elections, although they can certainly be spoilers for the party they're taking votes from. Remember 2000? I certainly do.