What factors contribute to the maintenance of the two party system?

One thing that we must not ignore is the voting system
that we have in the United States.  Our use of the "winner-take-all" system of elections
that we use.

In many other countries, a system called
"proportional representation" is used.  This means that parties are represented in a
parliament in proportion to the percent of votes they receive in an election.  In such a
system, a party that won 25% of the vote, for example, would get about 25% of the seats
in the legislature.

In the US, a party that won 25% of the
votes in an election would likely get 0% of the representation (unless there were very
odd circumstances with a large number of candidates splitting the vote).  In such a
system, third parties will have a tremendously difficult time breaking up the two-party
system.  Small parties cannot win any seats until they are able to win a plurality in a
given district.  This makes it hard for "third parties" to get going and helps to
maintain the two-party system that we have.