The two-party system is like the weather; everyone complains about it, but no one ever does anything to fix it. Not a presidential election goes by anymore without disaffected voters complaining about their choices. Fed up with the "Republicratic" (or Dempublican?) party, you don't have to go far before you find someone advocating for another option: Green, Libertarian, Constitution, Independent, or something even more obscure. But if the Democrats and Republicans are so universally reviled, why don't any of the alternatives do better?
Two-sided elections are easy. Everyone picks their favorite, and whichever side gets the most votes wins. But adding a third side complicates issues. Lets assume for starters that every voter is rock-solid in support of their current two-party pick, or at least--and this is more likely--rock-solid in their opposition to the other party's candidate. When a third option is added, every voter has a choice to make: stick with their current candidate, or change to the new one. Changing is dangerous because, even though they may like the third party a little better, they run the risk of a catastrophic outcome, throwing the election to that evil, horrible, opposition candidate!
Let's consider this three party election, or rather, all possible third-party elections. Imagine a triangle. Every point inside the triangle represents a different outcome in an election, each corner representing the outcome where 100% of voters chose the same party. Democrat on the left, Republican on the right, and Green off to the top somewhere. The further from that point, the less voters chose it.
When you put it all together you can see at a glance the result of any three-party election:
It all looks very nice and symmetrical. But let's remember, we're always starting at, or very near to, the bottom edge of the triangle, with almost no votes for the Green party. Now, let's assume that we're in a blue-leaning district. And we know that, predominantly, potential Green party voters are going to come from the Democratic party, and not from the Republican party. This change would be represented as a movement from a point on the bottom edge, up and to the right, parallel with the left side. Do you see what happens?
In trying to move to a more-preferred choice, these Republican-hating voters have accidentally given the election away to the party they hate! Now, you might say that these Democrats should just wait until there's enough of them to avoid clipping the Republican corner of the triangle. But for that to be safe, they would have to have more than two-thirds of the electorate on their side. Taking a quick look at Tuesdays highly-polarized election results, there are only two "states" where the leading party (plus all third-party votes) had a 2:1 or better margin over the losing party; Vermont and Washington D.C.. Among Senate races, only five had such lop-sided outcomes (one, Arkansas, had no Republican running). The point being that, if 3rd parties waited for such "safe" situations, they would have to wait for a very long time. The alternative is to try to gain so much support so quickly that they can "jump" over the offending party. But such fast changes are difficult to pull off on a local scale, never mind nationally.But I promised you a solution. The problem is this: if the winning candidate gets less than 50% of the vote, then the system fails.
Okay, that's not much of an answer. But there's not much else we can do; you and I know that most Green party voters would have otherwise picked Democrats, but are we sure they all would? We just can't know. Not for sure. And what if we also add a Libertarian candidate?
This is why these two reviled opponents will always have a stranglehold on American politics. Because a vote for a third party is a vote against their closest allies. Because a vote for something new also implies a vote for the greater of two evils. It's impossible to give them (by which I mean, us) what we really want, because by the point that it really matters, what we really want is no longer clear.
This is the essence of preferential voting. Asking for a voter's first choice is fine and dandy... but only as long as there are only two options. But with three options, asking you to pick one isn't enough for me to really know what you prefer. Imagine if I asked you "Would you prefer wine, beer, or some warm urine?" and when you said "Wine", I came back and told you "We're out of wine; I wasn't sure what you'd prefer otherwise, so I brought you the piss." By asking for voters to specify their alternate choices, we remove the element of fear and give third-parties the chance to grow and really compete in the market of political ideas, to offer us real options for change.