This is from Mother Jones and Gallop Poll. 

In any case, this really is a milestone. For a long time, one of the rocks of political analysis in America has been the simple fact that conservatives outnumber liberals. That's been true since at least the 60s, and probably for the entire postwar period—and it's been a perpetual millstone around Democratic necks. They couldn't win national elections just by getting the liberal vote and a little bit of the center-right vote. They had to get a lot of the center-right vote.

But it now looks like that era is coming to an end. With social issues increasingly defining politics, a social liberal is, for all practical purposes, just a plain old liberal—and the trend of increasing liberal ID is already underway. It's still got a ways to go, but the liberal-conservative gap is definitely closing.

The younger generation is more liberal and the conservatives are aging out.