When Americans vote, they almost certainly choose between only two parties: Republican or Democrat.
Since 1852, a candidate of the Republican or Democratic parties has occupied the first or second place in the US presidential elections, except for one. In the 1912 election, Theodore Roosevelt, a former Republican popular president, ran as president for a "third party" and took second place to Woodrow Wilson.
Before the Republican and Democratic parties were the two main parties, they were the Democratic Party and the Whig party. Before that confrontation, the Democratic Party and the Republican National Party were the two dominant ones. And before The Democratic-Republicans and the Federalists reigned.
Third parties have been minor participants in all US presidential elections appearing occasionally, but without being able to have a real chance of winning the presidency. It is also rare that they compete for seats in Congress, where, since the Second World War, no more than two of their 535 members have been from parties other than Republicans and Democrats. Among those exceptions is Bernie Sanders, the Vermont Senator elected to be part of the Congress as an independent and who is running this year for the presidential nomination of the Democratic Party.
This happens because the political system of the US is designed for two main parties, as it grants seats in Congress and the presidency with a method in which the winner takes everything. The candidates who run for Congress need only get the most votes to be elected. In 48 of the 50 states, presidential candidates get all of the state's electoral votes (which is the way a president is elected, state by state), as long as they get the most votes in that state.
Parties at risk of fragmentation will do everything possible to avoid candidates from a third party. When voters support the political ideals of a party but must choose between two candidates who defend those principles, the party will lose the elections since those candidates will divide the votes and allow the other party to obtain the majority.
Occasionally there are governors or senators of a third party, but often those parties have little general influence and find it difficult to become a national movement. Part of the problem originates in the difficulty of the game to win; another part of the problem is that the two main parties can make the qualification of candidates from a third party for a given election a real challenge.
While many independent candidates from other parties have run in previous elections, few received enough public recognition and even fewer received electoral votes in the states.
In addition to those candidates, the only candidate who did not campñana under the wing of one of the two main parties to gain access to a legitimate possibility of winning the general election was Roosevelt, who was a unique candidate in himself.
However, even in those times, the former president caused enormous tension by dividing the votes with his old party, the Republicans. He and his Republican successor as president, William Howard Taft, united to obtain the majority of the popular votes in 1912, but the Democratic candidate, Woodrow Wilson, won the presidency with the majority of the votes, which was less than 42%.
This supports why the two major political parties in the United States have an incentive to keep the system bipartisan.
