Why the US can only have two parties (OR: why democrats should support tea party candidates)

Every election day, people love to proclaim their frustration with this dirty-awful-horrible two party system, and insist that what we really need is a multi-party system.   And every election day, they're dead wrong.

Let's take the following hypothetical election with candidates A, B and C.  30% of the population votes for A, 30% for B, and the remaining 40% of the population of the C.

C is our victor.  The As and Bs will be upset, of course, but there are always winners and losers in every election.  We've done the best possible job at abiding by the will the people, and we've made as many people as possible happy.  Right?


It turns out that while A and B are individually less popular than B, they're very similar candidates.  Everyone who voted for B would have much rather had A over C.  The anything-but-C voters got split between A and B, and C ended up winning.  Put another way, if you only had two parties, only A or B would have run and people would have been happier.

The more parties the worse this problem becomes.

Potential 8 Party  Scenario:

W1 wins with 18% of the vote.   However, if we group people by similar values / goals / populations / whatever, we'd see that the L's have 42% preference, C's have 32% and W's are trailing with 26%.

But is this realistic? Yes.  Not only is it realistic, but it's already happened.  Remember 2000?

And if tea party candidates run separately, it'll happen again:

So next time someone is advocating for a multi-party system, remember this.  With its current election structure, the US can not support more than 2 parties. If we want more parties, then advocate for the necessary changes to the voting structure: rank order candidates, run off elections, etc..  But don't advocate for a multi-party system without those changes.