Election materials are set out in preparation for a primary election in Pennsylvania in April 2016. (Nabil K. Mark/Centre Daily Times via AP)

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When Americans go to the polls in November to elect their next president, it’s almost certain that they will be selecting between only two candidates: one Republican and one Democrat.

In fact, since 1852, a candidate from the Republican or a Democratic parties has placed either first or second in U.S. presidential elections, except for one. In that election, in 1912, Theodore Roosevelt, a popular former Republican president, ran as a “third-party” candidate, and he came in second place, losing to Woodrow Wilson.

And before the Republican Party and the Democratic Party were the two major parties, the Democratic Party and the Whig Party were. Before that matchup, the Democratic Party and the National Republican Party were the dominant two. And before that? The Democratic-Republicans and the Federalists reigned.

All the while, third parties have been small players throughout U.S. presidential politics, showing up occasionally but almost never having a real chance at winning the presidency. They also rarely compete for seats in Congress, where, since World War II, no more than two out of its 535 members have been anything other than Republicans and Democrats. Among those exceptions is Bernie Sanders, the senator from Vermont who was elected to Congress as an independent and who is running this year for the Democratic presidential nomination.

Why has this happened? The answer is that the U.S. political system is set up for two major parties, because it awards seats in Congress and the presidency with a winner-take-all method. Candidates running for Congress need only to get a plurality of the vote to be elected. In 48 of 50 states, presidential candidates get all of a state’s electoral votes — the way in which presidents are elected, state by state — as long as they win a plurality of the vote in that state.

French sociologist Maurice Duverger theorized in the 1950s that this kind of setup leads to what is effectively a two-party system. “Duverger’s law” states that third parties can’t compete because there is no prize for winning, for example, 15 or even 25 percent of the vote. This leads voters to choose candidates who are most likely to win, and it leads the parties to try to broaden their appeal to half of the electorate — and ideally more.

Parties at risk of splintering will do whatever they can to avoid third-party candidates. When voters favor a party’s political ideals but have a choice between two candidates who both support those principles, that party will lose the election because those candidates will split the votes, allowing the other party to win with a plurality.

There are occasionally governors or senators from a third party, but often these parties have limited influence overall and have a difficult time becoming a national movement. Part of this problem comes from the party’s difficulty in winning in the first place; another part of the problem is that the two main parties can make it challenging for third-party candidates to qualify for the ballot in a given election. (The United States, for example, allows each state to determine how a presidential candidate gets on the ballot. That means that third-party candidates generally have to be wealthy people who can fund their own campaigns and satisfy expensive requirements to get on the ballot in all 50 states.)

While many third-party and independent candidates have run for elections in the past, few have received enough public recognition and even fewer have received states’ electoral votes. Ross Perot, who ran as an independent, received 19 percent of the overall vote in 1992 but did not win a single electoral vote.

When such candidates get electoral votes, racial tensions are often involved. George Wallace (who won 46 electoral votes in 1968) and Strom Thurmond (who 39 electoral votes in 1948) were Southerners who ran as staunch opponents of integrating black and white Americans and are the last two non-Republicans and non-Democrats to win electoral votes. Similar regional third-party candidates caught on in the time of the American Civil War but never came close to actually winning.

Apart from that, the only candidate not running under the banner of one of the two major parties to have a legitimate chance at winning a general election was Roosevelt, who was a unique candidate unto himself.

Even then, though, the former president wound up badly splitting the vote with his old party, the Republicans. He and his Republican successor as president, William Howard Taft, combined to take a majority of the popular vote in 1912, but Democratic candidate Woodrow Wilson won the presidency with a plurality of the vote — less than 42 percent.

That reinforces why the two major political parties in the United States have an incentive to keep it a two-party system.