Indeed, we find that contemporary polarization is not only real — the ideological distance between the parties has grown dramatically since the 1970s — but also that it is asymmetric — congressional Republicans have moved farther away from the center than Democrats during this period. In two figures below, we plot the mean first dimension DW-NOMINATE scores of the two parties in the House and Senate from 1879 to the present. Since the mid-1970s, Republicans have moved further to the right than Democrats have moved to the left. This rightward shift is especially dramatic among House Republicans, from a mean of 0.22 in 1975 to 0.67 in 2012.
To be sure, political polarization is not entirely asymmetric. Congressional Democrats have moved slightly to the left during this period, but most of this is a product of the disappearance of conservative Southern “Blue Dog” Democrats. But the northern Democrats of the 1970s are ideologically indistinguishable from their present-day counterparts, with average scores around -0.4.
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