Political scientists have long noted that the growing polarization in Congress is asymmetric - both parties have drifted away from the center in the last 30 years, but Republicans have moved much farther than Democrats, particularly in the House.
The chart below, taken from the Brookings Institution's Vital Statistics on Congress, illustrates this quite nicely. It plots the average ideological scores of Republicans and Democrats in the House from 1947 to the present. The abrupt shift in the Republican trendline circa 1978 is unmissable.
Political scientists tend to suspect that this polarization among lawmakers is driving similar polarized views among the American public. Nolan McCarty, writing earlier this year in Monkeycage (home of some of the consistently finest writing on polarization on the web), notes that "since voters seem to be responding to the positions of their party leaders, the causal arrow seems to run from elite polarization to partisan sorting. Whether partisan sorting has an additional feedback effect on elite polarization is less clear."
A massive new Pew Research Center study on polarization in the American public gives some hints as to the direction of that causal arrow. It finds that the most consistently conservative voters are significantly more likely to say that they "always" vote in primary elections (54 percent) than liberal voters (34 percent). "But both of these groups are far more likely to vote than are people with a roughly even mix of liberal and conservative views, just 18% of whom say they always vote in primaries," the authors write.
Especially on the conservative side, much of the enthusiasm and energy during primary season comes from the strongest partisans. It stands to reason that these highly-politicized voters are driving the nomination of more ideologically extreme candidates on either side (see: Brat, David). And since the most conservative voters are significantly more engaged in the primary process than even their liberal counterparts, this may explain why Republican lawmakers are polarizing much more strongly than Democrats.
Here's another important point: despite their outsized enthusiasm for the primary process, the most liberal and conservative Americans make up relatively small shares of the American public - 12 percent and 9 percent, respectively. A plurality of Americans - 39 percent - falls squarely in the middle of Pew's ideological spectrum. But this group is by far the least likely to vote in primary elections, and in general elections too, according to Pew.
Because of their sheer numbers this group of mixed-preference voters could - should! - be the core of a centrist coalition. But because of their disengagement, their influence on the political process is diminished relative to the more partisan voices in the mix. This tells me that polarization may be driven as much by apathy at the middle of the political spectrum as it is by energy at the more raucous ideological ends.
Instead of a silent majority we have a silent plurality - and as Washington goes to war with itself, it's not paying attention.