Much of the move toward the ideological left has been driven by increased social liberalism among Democrats, according to Gallup. A majority of Democrats (53 percent) now call themselves socially liberal while just 35 percent said the same when Gallup first asked the question in 2001.
The movement leftward from Democrats corresponds with an even quicker move rightward among Republicans over the last decade or so. This graphic courtesy of Pew's amazing polarization study tells that story powerfully.
The most stunning stat out of the Pew study is this one: "Today, 92% of Republicans are to the right of the median Democrat, and 94% of Democrats are to the left of the median Republican."
There is some data, however, that suggests that Republicans may be moving back somewhat from the ideological outpost the party staked out earlier this decade -- even as Democrats have moved more rapidly to the left of late. This, from the same Gallup survey that produced the Democratic numbers cited above:
Beyond the topline numbers, there's other reasons in the Gallup data to suggest that the GOP may be in the midst of a re-adjustment toward the center. "An analysis of aggregated surveys conducted since the 2012 election shows that the size of the social and economic conservative group is twice as large among Republicans aged 65 and older as it is among those aged 18 to 29," writes Gallup boss Frank Newport. That's one of a number of data points that makes clear that younger Republicans are far less conservative, particularly on social issues, than older members of the GOP.
What does it all mean for 2016? That the fight for the Republican nomination might not, necessarily, be a fight to get to the ideological extreme. That Hillary Clinton is grappling with a different and more liberal Democratic party than when she ran in 2008. And that to the challenges to her ideological left from the likes of Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley may have more potential than originally thought. The Gallup data also makes clear why Clinton has worked to adopt more liberal policies in the run-up to her presidential bid -- from her full embrace of same sex marriage to her position on immigration reform.
The leftward movement of the Democratic party may well also complicate attempts by the eventual nominee to move toward the ideological center in the general election -- a problem that has been almost exclusively a Republican one in recent presidential elections.