Originally Posted by Blade

In the (non-parliamentary) congressional legislative system of the US, there is no party discipline - legislators can vote for whatever they want, without being expelled from the party as in parliamentary systems. That being the case, each of the two political parties are wide coalitions with members that in europe would represent many separate parties. Membership in one of the two major political powers therefore gives only a very rough indication of what a legislator supports - the record and character of individuals, as opposed to their party membership, is much more important in the US than in europe. All that being the case, opposition to the incumbent naturally coalesces, resulting in one party in power, and one out of power. There is nothing in the US Constitution or law prohibiting the existence of multiple parties. The coalition building that occurs in european parliaments happens here too - it just happens within the respective parties. Also, since a legislator does not operate under party discipline, he is much more likely to support what he views as the interests of his constituency, as opposed to the party line.