Hillary Clinton is the candidate of the Democratic Party's center-left establishment. Bernie Sanders is the candidate of its progressive, leftward wing.
That's certainly how the two candidates are running. But the exit polls in states that have voted so far show it might be a bit more complicated.
Among the questions asked by exit pollsters is whether voters want the next president to continue President Barack Obama's policies, be more liberal than Obama, or be less liberal. Consistently, Clinton wins those who want the next president to stay the course, and Sanders wins those who want the next president to move to the left.
But a fraction of the Democratic primary electorate — usually around 10 to 15 percent — wants the next president to be less liberal than Obama. Perhaps surprisingly, those Democrats tend to vote for Sanders, too.
It's important to take all exit polling with a big grain of salt. Exit polls are designed to quickly predict the winner of a state primary, not to represent the state's electorate perfectly. In some states, exit pollsters didn't even feel comfortable publishing how candidates did among this group of voters, because they didn't feel they had the data.
But in nine states so far, the exit pollsters did have enough data. And in all of those states, Sanders did better among Democrats who want the next president to be less liberal than he did among Democrats as a whole.
Furthermore, the difference was often pretty substantial. Sanders won "less liberal"-seeking voters in Massachusetts by 22 percentage points while losing the state by 1. He won Wisconsin by 14 points, but he won voters who want a "less liberal" president by 35.
What it means that Sanders is so popular with more conservative Democrats
In case you're wondering, exit polls do ask voters directly about their own ideologies. Clinton almost always wins self-identified moderates, and there are usually too few self-identified conservatives in the poll to break them out separately. When the exit polls have shown the voting preferences of self-identified conservatives, they've usually supported Clinton — but Sanders has consistently done slightly better among them than he has among self-identified moderates.
In Oklahoma, Sanders had a wider margin of victory among conservatives than he did among liberals. And in Massachusetts — in a reversal of the typical pattern — liberals supported Clinton, while moderates supported Sanders.
So why on earth is the rightmost wing of the Democratic Party unusually open to the self-proclaimed democratic socialist?
There's no way to know for sure, at least just yet. The exit polls don't offer many clues, and there hasn't been enough attention to the phenomenon for more rigorous study.
It's possible that despite their desire for a "less liberal" president, these voters simply agree with Sanders on the issues. Maybe they don't see his economic policies as "liberal." Maybe they see him as the candidate most likely to protect gun rights.
But it's also possible that the splits in the Democratic primary aren't entirely about policy. Perhaps the answer here lies in demographics. White Democrats are more likely to support Bernie Sanders than Hillary Clinton; conservative Democrats are overwhelmingly likely to be white.
Or perhaps Sanders has become the candidate of disaffected Democrats — and that includes both the people who are annoyed with Obama for being too moderate and those annoyed with him for being too liberal. Who Bernie Sanders is doesn't matter; the point is that he's not Hillary Clinton (or Barack Obama), and he's saying that the current path of the Democratic Party is not okay.
It's important to remember that a lot of people voting in Democratic primaries aren't necessarily following the ins and outs of the Democratic race. So it's definitely possible — in fact, it's probably likely — that the Democrats who want their party to move to the center don't think of Sanders as the candidate who is running to move it to the left.
In light of Sanders's success and perseverance in the primaries, many liberals are calling for "establishment Democrats" to think a little harder about how they engage with their base. But once they do, they might find that not all of their disaffected voters are disaffected for the same reason.