There was some question as to whether or not Bernie Sanders deserved credit for the spike. But it's worth remembering that Hillary Clinton won Iowa -- after finishing third eight years prior. A more liberal electorate, but one that rewarded Clinton with more of its votes.

As we noted at the time, there's another reason for the shift: Democrats have been describing themselves as liberal at increasing rates over the last two decades.

On Tuesday, Gallup released its latest numbers on how partisans see themselves. In 2008, only 36 percent of Democrats described themselves as liberal. In 2016, the figure jumped to 44 percent.

The result has been a narrowing in the advantage that "conservatives" have held in the population for some time. In 2009, 40 percent of Americans described themselves as conservative to 21 percent who called themselves liberal, a 19-point spread. In the most recent poll, that number is down to 11 points.

There hasn't been much change in how Republicans see themselves. So as Democrats increasingly shed the label "moderate" -- and "conservative" -- the overall density of conservatives and moderates drops.

The biggest shifts among Democratic voters since 2001 have been among young voters, whites and those with college degrees.

Those groups do overlap with Sanders' base of support, of course. It's worth noting that the percentage of nonwhite Democrats who identify as liberal continues to trail the rest of the party. We looked at this in March, as Sanders continued to struggle to attract black voters to his candidacy. Data from the General Social Survey since 1976 shows a marked drift toward liberal identity from white Democrats...

...but not among black Democrats.

Only a little over a third of nonwhite Democrats refer to themselves as liberal. It's a reminder that the advantage held by "conservatives" nationally doesn't necessarily translate into political dominance, since a healthy portion of the "moderates" vote heavily Democratic.