John Adams once wrote:

There is nothing I dread So much, as a Division of the Republick into two great Parties, each arranged under its Leader, and concerting Measures in opposition to each other. This, in my humble Apprehension is to be dreaded as the greatest political Evil, under our Constitution.

The US has only two major political parties. The Republicans and Democrats have consolidated their political power. They have the money, the media coverage, and the voters. This has been the case for most of our short history and will probably be the case for the next half century. At least.

That there are only two parties is built into the political system.

The US voting system is a plurality voting system. Voters can only vote for one candidate and the winning candidate takes all. There is no prize for second place.

Contrast this with proportional voting systems: partly list, mixed-member, or choice. In a closed party list system, parties present a list of their candidates. Voters cast their vote by party. Parties then win seats based on the proportion of their votes they win. If, in a five-seat race, a party wins 40% of the vote, their top two candidates will win a seat. In an open party list system, voters will vote by candidate rather than party. Of course, there are different methods which determine how the proportion of votes map to who will take office — the earlier example being a bit simplistic. What’s important is that there is a prize for second place.

Mixed-member systems are hybrids. In this model, half of the legislators are elected by plurality contest, the other half are elected by party list. We are assured that this is considered one of the hottest systems by those in electoral design.

Choice voting, or single transferable vote systems, ranks candidates by voter preference. A threshold of votes is set (by various possible mechanisms), and if a candidate meets that threshold and has more votes than their competitors, they are elected. If no candidate meets the threshold, then the last place candidate is eliminated and the votes for the last-place candidate are transferred to the next choice. This process is more complicated and depends on several implementation details, but is nonetheless interesting.

We’ll probably cover these alternatives later in more depth. Again, what’s important about these systems is that there is a prize for second place.

Most proportional voting systems offer parties a set of incentives that will not result in two parties. In proportional voting systems, one can win seats without winning the popular, electoral or whatever vote. It can be better for a party to win second place than to compromise or build coalitions with rival parties. Not so for parties in the plural system.

Suppose you reside in a country with a plurality voting system. You’re a politician looking to 
stay in office, err, save the world. There are three parties. The greens, the purples, and the yellows. The purples are coming in hot with 40% of the support. But the greens and the yellows, your very own party, are not doing so badly with about 30% support respectively. I mean, 30% isn’t so bad, but in your system, since 30% is less than 40% and winner takes all, it looks like you and your comrades will get 0% of the seats. So, you could take the complete loss, or compromise and build a coalition with the greens. Note that if you lose, there’s a good chance that the purples will bend the political rules in their favor, perhaps by re-drawing districts, leaving you waiting for the debate invite, or decreasing the probability that your favorite demographics would vote.

Voters in a plurality voting system will also experience the pressure to compromise. Instead of voting for their favorite candidate, they may judge that it will be better to vote for a bearable candidate who has a higher chance of winning. For example, though many voters may have preferred Stein to Clinton, they voted for Clinton instead — because she had a chance. The same goes for those voting in the Republican primaries, who may have preferred Kasich but were forced to vote for Cruz or Trump because of their higher chances of winning.

The force to compromise has historically been strong enough to pressure parties into fusing with others. As the Whig party lost its place as a major party, those remaining Whigs joined the Democratic party. Similarly for the People’s party, 50 years later. Apart from taking some votes from major parties, third-parties have been nothing more than a footnote for the last century.

This is Duverger’s law.

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