Why Do We Have a Two-Party System?


When you go to the ballot box to cast your vote for politicians, you see their party affiliation front and center. Republican, Democrat, Green Party, Libertarian: it’s right there, in bold letters. However, what some people might not know is that these political parties aren’t actually a part of the US government themselves. They’re private organizations with their own rules that aren’t laid out in the Constitution.

While these parties are bound by the laws that govern things like electoral processes and campaign funding, they’re not inherently part of the federal government. Let’s take a look at why this matters, and how we got to our current two-party system.

Duverger’s Law

The US government’s electoral system, as laid out in the Constitution, is a “single member district plurality”. That’s a fancy way to say that whoever gets the largest number of votes wins, whether or not they get the literal majority. So, if three candidates are running for a seat and two of them get 33% of the vote but one gets 34%, the one who got 34% gets the seat.

This is important because it leads to a two-party system. The two candidates who each lost by a percentage point are encouraged to join forces, bringing their voter bases together, and can hopefully form a coalition that will secure 66% of the vote in the next election. Under this system, parties that are big enough to bleed some votes away from popular parties will inevitably be subsumed by larger parties.

This is called Duverger’s Law, and it’s why the US has a two-party system. So, whether or not Republicans and Democrats are doing good jobs in their roles, odds are good that those two parties will be in control of most government positions.

Are There Alternatives?

The US’s system isn’t the only way to elect representatives. Many countries use parliamentary systems, where the number of seats a party gets is proportionally related to how many votes they get. So, in the above example, two of the parties would have 33% of the seats, and one would have 34%. This results in smaller parties being able to have their voices heard, though some accuse this system of increasing instability by giving rise to vocal minorities within the government.

There is no “right” way to set up a form of government, however. So, while the Republican and Democratic parties aren’t actually part of the government in a legal sense, the reality of the way the US selects it representatives makes them the de facto two-party system of the country.