But … I’ve been reliably informed by lefty media over the past five years that tea-party radicals are the exclusive drivers of ideological polarization. While the GOP drifts ever further rightward, Democrats remain stubbornly stuck in the sensible nonideological middle.

The new theory, I assume, is that these tea-party radicals have so angered reasonable Democrats by vacating the sensible center that they’ve redefined that center as “liberal.”. Wingnuts: Is there anything they can’t do?

My instinct on first looking at that graph, especially the initial spike in 2003, is that this was driven by Iraq. That seems illogical given that it’s a measurement of social and economic sentiment, not foreign policy, but politics is often illogical. Iraq sharpened Democratic opposition to the Republican-run government and that opposition bled over into how they self-identified even on domestic policy. It became respectable, at least on the left, to be a loud and proud liberal again. I think there’s something to that, but there’s got to be more. My next instinct was that this is a function of Warrenism and the growing lefty obsession with income inequality, and there’s surely something to that too. But maybe not as much as we think: When you ask Dems how they identity on economic issues now versus in 2001, the share who say “liberal” is up only nine points.

The real gains are in social issues. Behold:

A whopping 18-point increase since ’01, better than a rate of one point a year. It’s got to be gay marriage (and, to a lesser extent, support for marijuana legalization) that’s driving that, right? As support for SSM has risen, so has the number of people willing to identify as socially liberal. And now that it’s a majority position in the U.S., Dems who may have resisted identifying that way before feel no compunction about doing so. That makes me wonder if the key difference between social issues and economic issues on the left isn’t so much that fewer Dems support Warrenism than support gay marriage than that economic liberalism still carries a degree of stigma after the 60s and 70s that social liberalism doesn’t at the moment thanks to the left’s recent culture-war victories. It may be that Dems who are worried about income inequality and favor left-wing populist solutions have recast that as the “moderate” position, to avoid any lingering stigma from centrists, whereas they’re happy to claim SSM as a liberal victory since the stigma there has all but eroded outside the right. Right or wrong? Theories welcome.

Update: Fair point:

@allahpundit The reason this is going up is because both parties are bleeding moderates and independents http://t.co/FCaVosyWuF

— 🇺🇸 Vlad3060 🇨🇺 (@vlad3060) June 18, 2015

As centrists in both parties switch to independent in disgust, all that’s left are the ideological rumps. There’s a chicken and egg problem there, though: Are Democrats skewing left because centrists are leaving, or are centrists leaving because more Dems are skewing left? There’s not mutually exclusive but I’d like to know which effect is greater.