What’s interesting about this is that the change is due almost entirely to Democrats and Democratic leaners. Since 1999, that group has gone from 35 percent socially liberal to 53 percent, and from 20 percent socially conservative to 14 percent conservative.
Republicans and Republican leaners, by contrast, have barely budged. In the 2015 polling there’s a slight dip in conservative ID and a slight spike in moderate ID, but it’s probably just noise. Generally speaking, the lines are pretty flat over the past couple of decades.
So why have Democrats changed so much? Perhaps it’s the impact of Millennials. Perhaps it’s the impact of gay marriage, which Democrats have been far more willing to accept than Republicans. Maybe MSNBC and liberal blogs have had a bigger impact than I would have guessed. I’m not sure. But the increase has been steady enough that it can’t be blamed on any specific event, like the Bush presidency or the financial crisis.