“It’s become commonplace among observers of U.S. politics to decry partisan polarization in Congress. Indeed, a Pew Research Center analysis finds that, on average, Democrats and Republicans are farther apart ideologically today than at any time in the past 50 years. But the dynamics behind today’s congressional polarization have been long in the making. The analysis of members’ ideological scores finds that the current standoff between Democrats and Republicans is the result of several overlapping trends that have been playing themselves out – and sometimes reinforcing each other – for decades.

• Both parties have grown more ideologically cohesive. There are now only about two dozen moderate Democrats and Republicans left on Capitol Hill, versus more than 160 in 1971-72.
• Both parties have moved further away from the ideological center since the early 1970s. Democrats on average have become somewhat more liberal, while Republicans on average have become much more conservative.
• The geographic and demographic makeup of both congressional parties has changed dramatically. Nearly half of House Republicans now come from Southern states, while nearly half of House Democrats are Black, Hispanic or Asian/Pacific Islander.

The Center’s analysis is based on DW-NOMINATE, a method that uses lawmakers’ roll-call votes to place them in a two-dimensional ideological space. It is designed to produce scores that are comparable across time. This analysis focuses on the first dimension, which is essentially the economic and governmental aspects of the familiar left-right spectrum and ranges from 1 (most conservative) to -1 (most liberal). (For more details on DW-NOMINATE and this analysis’ geographical definitions, read “How we did this.”)