In the United States, since roughly 1800, there have been two major political parties. For the last 150 years or so, these two parties have been the Democrat and Republican parties. This is in deep contrast to many other democratic nations, who often have as many as ten significant parties. Many people wonder why the United States has only two parties and why it is so difficult for a third or a fourth party to make any significant gains and become a noteworthy player on the national political scene. The reasons are many and varied.
The first two opposing groups in U.S. political history were the Federalists and the Anti-Federalists. In the early years of our nation, the Federalists dominated in power, because they were essentially the "money" party; they represented the merchants and the manufacturers, whereas the Anti-Federalists (who would gradually become known as the Democratic Republicans, and eventually as just the Democrats) represented the artisans and the farmers.
Eventually, these two parties evolved into representing two distinct sets of interests. The Democrats came to represent the agricultural and frontier interests in the South and West, while the Federalists (who eventually evolved into the Republican party) supported commercial interests in the East. In other words, certain parts of the country voted almost exclusively for one party, and the two parties held ideologies that best supported their particular section. This was known as sectional politics, and the idea lasted well into the 20th century.
Given their already established power, the two party system managed to perpetuate itself onto a national level, this time making its power on the idea of class politics, where traditionally the Republican Party represents the more financially affluent and the Democratic Party represents the working class and the poor. In other words, the two parties have always benefited by exploiting geographic and financial divisions.
Children often adopt the political party of their parents. Given that throughout history, the two parties managed to make themselves dominant through sectional and class politics, their power is self-perpetuating through the fact that many children simply inherit the political party of their parents, guaranteeing the two major parties a built-in voting bloc with each generation.
Another significant reason for the perpetuation of the two party system is the fact that most Americans have common goals. Most Americans desire material prosperity, and they believe it should be gained through individual initiative. There hasn't been much support for collectivist ideas; as a result, socialist and communist parties have found it very difficult to gain a foothold in the United States.
Another reason that there haven't been more parties to gain success is that our nation has a strong separation of church and state. As a result, there have never been strong parties representing various religious ideologies, nor is there a need for them.
The real division in the United States is economic, and both ends of the economic spectrum are represented by the current parties. Since this is the primary issue of concern in the United States, it becomes very difficult to fit another party into the culture.
Also, special interest groups are only willing to donate money to parties that can get officials elected. It is not in their best interest to donate to parties that can't help their ideas in terms of legislature, so their campaign support usually targets candidates from major parties, giving them a financial edge.
The Electoral System
Rather than use a representative system where if a certain percentage of the populace agrees with one party, that party is represented with that percentage in Washington, the United States uses a winner-take-all election scheme. The winner of an election goes to Washington and takes his or her seat; the loser gets nothing. If, in a presidential election, a state is carried by one candidate, that candidate gets all of that state's votes in the electoral college, where the president is formally elected.
Because of this winner-take-all system, it is almost impossible for third parties to build up support. It is very difficult for a third party to get a member elected to a statewide office (but not impossible). As a result of this inherent difficulty, the minor parties have no way to gain any significant ground in legislatures or executive offices, and thus never have the advantage of incumbency or the ability to demonstrate their party's platform at work.
State & Federal Law
As if the other factors weren't enough, there are many state and federal laws in place that guarantee that the current two-party system remains in place. In many states, the two major parties don't need nearly as many signatures as the minor parties to get a candidate on the ballot for a particular office, making the road to even getting their name on the ballot much longer and harder for a minor party.
On the federal level, there are different obstacles. Most of the power in Congress is divided up by party membership, so even if a member of a minor party were to be elected, he or she would be either completely powerless or would have to join (or at least affiliate with) one party or another. Also, the FEC (Federal Election Commission) funds parties based on their performance in previous elections, meaning that candidates from new parties do not receive equal funding until they can match the performance of candidates from the two major parties, placing another financial roadblock in the way of a new political party.
Unless there is a very significant change in the thinking and ideology of the American populace and there is a significant change in the legal structure of politics in the United States, it seems clear that the two party system is here to stay. The best thing one can do to help a third party get started within this system is to volunteer some time to help raise money and get candidates elected on a local level; in other words, help get the ball rolling on a grassroots level.