Idealogical scores of members of Congress since the 1970s.
Pew Research Center
study / power

The Polarized Congress of Today Has its Roots in the 1970s

Polarization in Congress began in the 1970s, and its only been getting worse 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.”

The researchers aggregated roll call votes to locate each member of Congress, from 1789 to the present day, on a two-dimensional grid. One dimension represents the traditional liberal-conservative spectrum; the second picks up regional issue differences, such as the split between Northern and Southern Democrats over civil rights in the 1950s and 1960s. As Poole and Rosenthal note, those formerly significant regional distinctions have declined in importance — or, more precisely, merged into the overall liberal-conservative divide: “Voting in Congress is now almost purely one-dimensional — [political ideology] accounts for about 93 percent of roll call voting choices in the 113th House and Senate.” So we used just the ideological dimension in our analysis.
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