Matthew Yglesias, linking to Noam Scheiber, points out that America's frenetic, famously 'polarized' political debate is not actually backed up by real policy polarization. In other words, the two sides trade blows furiously, and then in practice follow policies that are not wildly different from each other (Bush increased social spending, Obama looks after financial interests, etc).
With Mark Blyth and Riccardo Pelizzo, I've been studying the increasing 'cartelization' of party systems in western democracies - the reluctance of the mainstream parties to actually compete over the broad contours of economic and social policy. The US is one of the neatest examples of this, with the Republicans tracked fairly closely by the Democrats in their shift to the right over the past quarter century. This table maps party positions on the left-right scale during presidential election campaigns over the post-war period (positive numbers on the vertical axis mean right-wing positions, negative mean left):
There's a hint of a growing divergence in the mid-2000s, but what's most striking is how the Democrats have drifted rightwards over the post-war period. Evidence assembled by Bartels, Poole and Rosenthal and others neglect the substantive political convergence that has taken place whilst partisan polarization in Congress and public opinion have been developing.
In view of recent events, it would seem particularly grim if the purported polarization was in fact a facade concealing substantial agreement on the same set of - largely conservative - policy positions.