The polarized Congress of today has its roots in the 1970s

http://www.pewresearch.org/fact-tank/2014/06/12/polarized-politics-in-congress-began-in-the-1970s-and-has-been-getting-worse-ever-since/

You don’t have to look hard to see evidence of political polarization – just watch cable news, listen to talk radio or follow social-media debates. Indeed, a new Pew Research Center report finds that Americans are more ideologically polarized today than they’ve been in at least two decades. Their representatives in Congress are divided too, and have been pulling apart since the days of M*A*S*H and Billy Beer.

With Democrats and Republicans more ideologically separated than ever before, compromises have become scarcer and more difficult to achieve, contributing to the current Congress’ inability to get much of consequence done. But going beyond anecdotal evidence to examine congressional polarization more rigorously can be tricky.

Fortunately, political scientists Keith Poole and Howard Rosenthal have developed a widely accepted metric, DW-NOMINATE, that places every senator and representative on the same set of ideological scales. Using their data, it’s clear that the congressional parties, after decades of relatively little polarization, began pulling apart in the mid-1970s. Today, they say, “Congress is now more polarized than at any time since the end of Reconstruction.”