The United States is a typical example of a two-party system. The Democratic and the Republican parties have dominated politics since the Civil War. No third party has won a Presidential election or majorities in either house of Congress.
There are two main reasons for this situation. The first reason is the fact that these two dominant parties have expressed a position in almost all aspects of politics and society. Moreover, these views are often opposed to each other. This leaves little room for a third party to arise.
Another reason why these parties have been so durable is the fact that smaller parties often face a lot of obstacles when trying to compete with larger parties, such as denial of access to general election debates. This makes it difficult for them to be taken seriously and to spread their message, which also reduces the number of people that they can access.
