What are the main factors that contributed to establishment of America's two-party system?

I will point
to two reasons why the United States has a two party system.

First, the two
party system came about because of America's electoral system.  In the United States, we have
what are called single-member districts with "first past the post" elections.  In
other words, each district elects only one representative -- the person who gets the most
votes.  This is as opposed to a proportional...