But why did they change? Here we reach the nub of the matter: The ideological composition of the electorate shifted dramatically. In 2006, those who voted were 32 percent conservative, 47 percent moderate, and 20 percent liberal. In 2010, by contrast, conservatives had risen to 41 percent of the total and moderates declined to 39 percent, while liberals remained constant at 20 percent. And because, in today’s polarized politics, liberals vote almost exclusively for Democrats and conservatives for Republicans, the ideological shift matters a lot…

This shift is part of a broader trend: Over the past two decades, moderates have trended down as share of the total electorate while conservatives have gone up. In 1992, moderates were 43 percent of the total; in 2006, 38 percent; today, only 35 percent. For conservatives, the comparable numbers are 36 percent, 37 percent, and 42 percent, respectively. So the 2010 electorate does not represent a disproportional mobilization of conservatives: If the 2010 electorate had perfectly reflected the voting-age population, it would actually have been a bit more conservative and less moderate than was the population that showed up at the polls. Unless the long-term decline of moderates and rise of conservatives is reversed during the next two years, the ideological balance of the electorate in 2012 could look a lot like it did this year.