[Coda]

Yes, but that's beside the point. There were only (practically speaking) two candidates who could receive votes. All voting systems behave the same way when there are only two candidates.

The point here is that single-vote ballots prevent a third-party candidate from having a chance at all, which is why there WERE only two possible outcomes to this election.

Suppose instead of just Barack Obama and Mitt Romney, Joseph Schmo was also on the ballot. Mr. Schmo has the same platform as Obama with regards to social issues, but he has a unique fiscal platform that doesn't align with either of the other platforms.

Most voters who like Mr. Schmo say that if Joseph can't win, they'd rather see Obama win than Romney.

In a single-vote ballot, unless Mr. Schmo can gather enough votes to win outright, every vote for Schmo is a vote that isn't going to Obama, so the social-liberal demographic is divided among two candidates while the social-conservative demographic is unified under Romney. So if 35% of the population votes for Obama, 25% of the population votes for Schmo, and 40% of the population votes for Romney, Romney wins, despite the fact that 60% of the population would rather see Obama win than Romney.

This means that third party candidates are better off supporting one of the two major candidates and then working from outside the system (as lobbyists) to ask their preferred candidate to consider their agenda.

A ranked-preference system such as IRV or pairwise gives third-party candidates a chance.

Let's look at IRV: In the previous example, if everyone who voted Schmo ranked it as "1. Schmo, 2. Obama, 3. Romney", then Schmo would still lose (getting 25% of the vote means he would get ejected in the first round of instant runoffs), but Obama would win in the end -- there's no paradox here where a vote for your most preferred candidate ends up with your least preferred candidate winning.

But here's the thing: It means that Schmo doesn't have a reason not to run, so he can TRY.

And if those numbers were slightly different -- say 31% voted for Schmo and 29% voted for Obama -- then Schmo would win, because that's still 60% of the population that doesn't want Romney.