Reader NS asks “can you recommend any good works about why the two party system is so entrenched here? There doesn’t seem to be any good constitutional reason — just historical tradition.”
I think it’s the confluence of three different elements of our institutional structure:
— The first element is Duverger’s Law, which explains that when you have single-member constituencies and victory-by-plurality you tend to get two-party competition. Once it becomes clear to voters that the winner of a contest is overwhelmingly likely to be either Candidate A or Candidate B, then people who may like Candidate C or Candidate D better will tend to vote tactically for A or B.
— Second, there’s the presidency. Even if most races decided by plurality wins should come down to two parties, you could still have a bunch of parties across the face of a country. Thus in the UK you have some seats where Labour and the SNP are the major parties, some seats where the Tories and the Liberal Democrats are the major parties, lots of seats where it’s Labour and the Tories, plenty of safe seats where one party dominates and nominal opponents divide the residual minority, and relatively few genuinely three-sided races. But in America we have a presidential election which creates incentives to try to construct a national coalition in a way that discourages that dynamic.
— Third, there’s weak party discipline. If the House Blue Dogs were subjected to the kind of very tight party discipline that exists in many countries, they would have a strong incentive to try to form some kind of independent political organization. But since US politics features weak discipline, it’s easier to stay within a party coalition and then form an intra-party factional organization.
— Fourth, there’s lock-in. Election rules are made by Democrats and Republicans. Consequently, the rules disadvantage anyone who’s neither a Democrat nor a Republican.
One of the main trends in American politics is the diminishment of factor three and the emergence of tighter party discipline. The Republican Party is still laughably undisciplined by international standards, and the Democratic Party even less disciplined, but the trends are pointing toward stronger parties. There’s a lot to be said for strong parties, but they don’t necessarily suit the rest of the American political system very well. It’s hard to have a party that’s both tightly united and also tries to appeal to a majority of voters in a continent-sized country of 300 million people.