In his Huiffpo post “Can Democrats Avoid the Circular Firing Squad?,” Robert Kuttner, cofounder and co-editor of The American Prospect discusses one scenario for an upset win of the Democratic nomination:

…Hillary Clinton could still lock up the nomination by the last primaries on June 14, but not without relying on super-delegates. Here are the numbers:
Clinton has 1,769 pledged delegates won in caucuses and primaries, out of 2,310 delegates required for nomination. There are 913 yet to be awarded in the last round of primaries. To go over the top before the convention, not counting super-delegates, Clinton needs to win 541 more delegates, or well over half. But with Sanders surging nearly everywhere, that seems extremely unlikely.
So the state of play after the six states vote June 7 (DC votes June 14, but has only 20 delegates) is likely to show Clinton with 50 to 100 votes short, Sanders with momentum, and the Sanders campaign mounting a last ditch effort to persuade most of the 712 super-delegates (541 of whom have already declared for Clinton) to reconsider, on the premise that Sanders has the better shot at beating Trump.

I’ll leave it to others to analyze this delegate math. But the nightmare scenario for Democrats would be if one of the two candidate wins the popular vote majority, while the other wins the delegates needed to win the Democratic nomination. No matter which candidate is nominated under those circumstances, it would be tainted, perhaps fatally.
It is the popular vote that confers moral legitimacy on a candidate. That’s one reason why the Bush II presidency will always be viewed as a failure of democracy, and one which led to horrific consequences.
If Clinton wins the popular vote but loses the nomination, many of her supporters will call it out as yet another example of systemic denial of women’s rights, and not a few will stay home on election day. Some may even write her in.
If Sanders wins the popular vote, but not the necessary delegates, many of his supporters may stay home on election day, vote for a write-in or third party candidate or, worse, support Trump as a protest.
Either one of these “winning ugly” scenarios will cast the dark shadow of the ‘Dems in Disarray’ narrative over the election, and dramatically reduce the possibility of a Democratic victory. It would almost certainly gut hopes for a Democratic landslide that extends down ballot.
It’s possible that separate winners of the Democratic popular vote in the primaries and delegates would not necessarily lead to a Trump presidency, and that a Democrat could win. Trump in the White House is such a frightening prospect, that a Democratic nominee just might be able to win without having first won a majority of the party’s primary votes. But that’s a pretty high-stakes gamble.
At present Clinton leads Sanders by about 3 million popular votes. It would be a tall order for Sanders to finish with more popular votes in the Democratic primaries, but it could happen. He has some momentum.
But, if Sanders and Clinton made a mutual pledge to ask their delegates to support the candidate who wins the most popular votes when the primaries and caucuses are all finished, it would affirm the Democratic Party’s commitment to democracy and enhance Democratic voter solidarity. it would show that both Democratic candidates support the will of the people over super delegate politics.
It’s really not such a radical idea. The super delegate system is a train wreck in waiting. It should be dumped at the earliest opportunity. But both candidates can render it harmless right away with a popular vote pact that doesn’t require a rules change.
Polls indicate that Sanders and Clinton can both beat Trump, assuming Democrats unify behind their nominee. A popular vote pact between them could promote Democratic unity. It’s a good choice both Democratic candidates can make with little or no downside, and the timing is about right.