Why there are only two major political parties in the US

With the US elections coming up soon, have you ever wondered why there are only two major parties? Is it just a coincidence or is there something more fundamental going on? It fact it is nearly inevitable that the kind of voting system used in the United States will lead to a two party system. This will happen in any election which has the following characteristics:



  1. Voters have a single vote
  2. They vote for a single candidate in their district
  3. The district has one legislative seat available in the election
  4. The winner is determined solely by who has the most votes

Tactical voting

What happens in this type of election when a third party enters is that it makes voters start to think tactically.

Imagine that there are two parties, one right-wing and one left-wing and that in the election the right-wing party is predicted to get 55% of the vote to the 45% for the left-wing party. The right-wing party wins (please don’t be offended if you would want the left-wing party to win, they will in a moment!)

Now a third party decides to join the race, it is a more extreme right-wing party and 20% of the voters agree with the policies of this new party. If everyone now voted for the party whose policies they believe in then the left-wing party still gets 45% of the vote, the moderate right-wing party gets 35% of the vote (55% less the 20% that move to the more extreme party) and the extreme right-wing party gets 20% of the vote.

This means that the left-wing party wins as the right-wing vote is split.

In practice voters realise that voting for the extreme party, although in line with their beliefs, would let in the left-wing party that they really don’t want to win. To avoid this they stick with the moderate right-wing party that they believe can win the election and is closest to their beliefs of the two main parties.

Unless a third party can convince enough voters that they really have a chance to win overall then voters won’t switch and the two parties will continue to dominate.

This idea was first put forward my Maurice Duverger and is now known as Duverger’s law.

Image: FreeDigitalPhotos.net

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7 Responses to Why there are only two major political parties in the US

  1. Trader says:

    What this simplistic model neglects to evaluate is that the standard party might move in a direction you as a voter are not comfortable with. So regardless you(ets assume you are right wing) will vote for a different party; maybe the left wing, maybe the 3rd party. The ideas of a party and their elected candidate are not static.
    Also this example assumes that only one party (right wing) is willing to vote for the other 3rd party candidate while in reality they should pull in from both sides.

    • Barry Hughes says:

      Hi Trader,

      The model is certainly a simplification of a real situation but the principle still holds that generally you will end up with two parties with this kind of voting system.

      Of course it isn’t always the case, the UK has a similar voting system but maintains three main parties and a number of regional ones as well. I am doing some analysis of the UK general election in 2010 which I hope to post in the next week or two.


  2. John says:

    But aren’t the criteria that you mentioned exactly those that are in effect in Canada. And yet Canada does not have a two-party system. In fact, often times, we see more than one party with significant presence in the house of commence.

    • Barry Hughes says:

      Hi John,

      You are right that Canada, like the UK, is a country where more than two parties are able to thrive. This tends to be because of regional differences, so some parties are strong in certain parts of the country, but not others.

      I think that most of the time individual constituencies become two party races, even if there are different parties competing in different parts of the country. For example, in the UK the Liberal Democrats are strong in the south-west of the country, so constituencies will often be fights between the Conservatives and the Liberal Democrats with the Labour party getting very few votes. In another part of the country the Liberal Democrats will be a lot weaker.

      I am analysing the UK situation in more detail and hope to post some more detail soon.

      • John says:

        Hello Barry,

        Thanks for your reply. I am looking forward to reading your post on the UK situation.
        On a similar note I have to add, although in Canada we usually see more than two parties at the house, the notion of vote split on the right or left of the center is spoken of regularly. Some people believe the majority government that was handed to the conservatives last year was due to a split in the votes in the left of the center, whose support is shared between the liberals and the NDP.
        Perviously, there used to be two parties on the right and two parties on the left. After a merger between the two parties on the right, we are faced with a situation where there is one party on the right and two on the left. This has, usually, resulted in the conservatives benefiting from the vote split between the NDP and the Liberals.

  3. Kasina says:

    Hi Barry, as readers before me pointed out, this model is simplistic. This model fails to recognize that unlike in the US, party ideologies aren’t defined just as “right-wing” and “left-wing”. According to me, the correct conclusion that should be drawn from your model is that the number of parties will be approximately same as the number of ideologies. Take a country like India for example. 29 states-many religions-many minorities. Each group feels that their interests are not being properly represented and to rectify this, will vote for them. Who forms the government is inevitably decided by who can form the biggest coalition by mutually compromising on certain issues. On a side note, I feel that is a more proper form of democracy (than the system we have here), where the voter has many options to choose from 🙂

  4. Mari Vega says:

    So how do we break this see-saw, either/or stranglehold? Neither party represents me fairly, nor will they ever because they only want to perpetuate their stranglehold to as many levels of government as possible. Did you know that judges in lower courts who are elected declare one of the two party affiliations? And, if it isn’t widely known, because our elections are run and won by whomever spends the most money, they usually win the most offices.

    Really, how do we get off this nightmarish merry-go-round?

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