Since 1856, two political parties have been dominant, the Democratic Party and the Republican Party. As they have butted heads, no other party has been elected to the office of president. In fact, the only time a third party received more votes than the Republican or Democratic parties was in the election of 1912. Why has there only been two parties that have dominated our government for nearly a century and a half? Because the two-party system works.
The two-party system makes voting for Americans a lot simpler. If the voter is conservative, and he doesn't know much about certain candidates, he could just vote for the Republican candidate. It combines groups of people with similar platforms into two solid platforms. The Republicans are conservative and the Democrats are liberal. The voter can choose either side.
Sometimes, the voter can choose neither side. This happens when a party becomes split and a third party forms. When this happens, the voters split along with the parties. It can cost an election, such as the 1912 split of the Republican party when Theodore Roosevelt formed the Progressive party. In this year's election, the Democratic nominee Al Gore is worried that the Green party candidate, Ralph Nader, will "steal" votes from him in the presidential election. Other third parties, such as the Green party in Alaska and the Independence party in Minnesota, have an impact on the state and local levels, but not on the national level.
Of course, there are alternatives to the two-party system. One of the most successful alternatives is called proportional representation. Under this system, the number of representatives in the government is determined by the strength of the popular vote. For example, if ten percent of voters vote for a third party, that party will hold ten percent of the representatives. This worked quite well in West Germany before its unification with East Germany.