Every party system in the United States - there have been several [0] - has been a duopoly. This is a natural outcome of a first past the post, single representative per district electoral system (see also: the UK).

The extremity in current politics - and among elected politicians wielding power, such is to be found much more on the right - is in part a function of two trends: the gerrymandering of those districts, and the tendency for states and areas to become more ideologically homogenous. (I’m not sure of the latter’s root cause - perhaps internal migration or new media consumption patterns.)

Both of these trends tend to shift the political competition in those districts, be they states or house districts, towards purity tests over compromise. One wins a primary by being the most extreme; if the primary matters more than the general, extremity increases generally.

If you want to reduce extremity, fight partisan redistricting.

0 - https://en.wikipedia.org/wiki/Political_parties_in_the_Unite...