As we've noted before, Sanders has been as successful as he's been in part because of voters just like him: People that vote Democratic but who identify themselves as independents when asked by exit pollsters. There's a direct correlation between the percentage of the electorate that identifies as independent in Edison Media Research exit polling and how Sanders has fared in the state's primary.
But there is a more striking way to present that success. In states for which we have exit poll data, people who identify as independents have voted for Sanders over Clinton 23 out of 26 times. (The exceptions: Georgia, Mississippi and Alabama.) People who identify as Democrats, meanwhile, have voted for Clinton over Sanders in 23 out of 26, excepting Vermont, New Hampshire and Wisconsin, where the two tied.
For all of the talk of the split in the Republican Party -- a very real split centered around their brand-new nominee -- this split among the Democrats is perhaps more immediately obvious. It's almost certainly not as dangerous for the party, given the regularity with which Sanders supporters say they'll support Clinton if she's the party nominee. On policy matters, the two groups largely agree (which is why those independents vote with the Democrats anyway). It's not a Donald-Trump-vs.-the-establishment fight, where Trump severely undercuts key policy positions. So the Democratic divide may not be as dangerous as the Republican rift, but it's much easier to spot.
Since these are self-identified independents, though, we have a chicken-and-egg question. Are Sanders backers more likely to say they're independents, driving up the numbers?
It doesn't seem that way. In May 2008, Gallup estimated that around 36 or 37 percent of the country identified as Democrats and about 16 percent were Democrat-leaning independents. In the most recent survey, conducted at the beginning of April, the figure for Democrats was 31 and independents, 18. On average, in states where we have exit poll data, the change in the number of self-identified independents over 2008 has been an increase of 2.1 percentage points. In Oklahoma and North Carolina, the increase in the number of independents was 12 and 9 percent, respectively -- two of the biggest increases. But those states were split between Sanders and Clinton.
It's likely that there's overlap here with Sanders's other big group of supporters: Young people. Younger voters are much more likely to identify as independents but support Democrats than are older generations.
Why isn't Sanders conceding? There are two Democratic primaries happening. Hillary Clinton has locked up the Democratic party nomination, largely because there are still far more Democrats who vote in the primaries than Democrat-leaning independents. If there were a separate nomination for those independents, Sanders would win in a landslide.
Unfortunately for Sanders, there isn’t.