Clinton currently has 2,313 total delegates -- 1,769 of which are pledged and 544 of which are superdelegates -- and she is expected to cross the 2,383-delgate threshold in the next few days to clinch the nomination. But Sanders, who has 1,501 pledged delegates and only 46 superdelegates, says he can still woo enough of her superdelegates between now and the Democratic convention in July to swing the nomination his way.
It's a tall order.
Pledged delegates emerge from primaries and caucuses, while superdelegates are party leaders -- elected officials and former ones who have individually committed to a candidate. It would be unprecedented for the number of superdelegates Sanders needs to switch allegiances, and, like Clinton this year, then-Sen. Barack Obama entered the 2008 convention without a majority of pledged delegates.
Sanders is making this pledge to keep his fight alive in the closing days of the California primary campaign, sending a signal to his supporters that the race isn't finished.
"The media is in error when they lump superdelegates with pledged delegates. Pledged delegates are real," Sanders said. "Hillary Clinton will not have the requisite number of pledged delegates to win the Democratic nomination at the end of the nominating process on June 14. Won't happen. She will be dependent on superdelegates."
He vowed, "The Democratic National Convention will be a contested convention."
Yet that decision isn't entirely his alone. If enough superdelegates pledge their support to Clinton -- and President Barack Obama, Sen. Harry Reid and other party leaders weigh in next week -- Sanders will face new pressure to reconsider his fight.
The Vermont senator accused the media of lumping together pledged delegates and superdelegates, noting that superdelegates don't formally cast their votes until the convention in late July, or, as Sanders put it, "six long weeks from today."
Sanders, however, acknowledged that it's unlikely he'll be able to turn around his fortunes.
"We understand that we have a steep climb," Sanders said. "I'm not here to tell you that tomorrow we're going to flip 300 superdelegates. You don't hear me say that. But I am saying we are going to make the case."
At a rally outside the Los Angeles Memorial Coliseum Saturday night, Sanders fired up the crowd by repeating his pledge to go to the convention, citing his performance in polls with him and Trump.
"And what I hope that the delegates going to the Democratic National Convention understand is that in virtually every state poll we do much better against Trump than does Secretary Clinton," he said.
Clinton won the caucuses in the Virgin Islands
Saturday, and Puerto Rico holds its primary on Sunday, when Clinton will likely get her earliest opportunity to clinch the nomination. California, Montana, New Jersey, New Mexico, North Dakota and South Dakota hold their primaries on Tuesday. Washington, D.C., holds the final nominating contest of the primary season on June 14.
Superdelegate process under scrutiny
For months, Sanders and his campaign have railed against
how the Democratic National Committee and Chairwoman Debbie Wasserman Schultz have handled the primary process, claiming it has helped Clinton with debates held on Saturday nights, closed primaries in major states such as New York, and the use of superdelegates.
"That is called an anointment process, not a democratic process with a small or large d," Sanders said on Saturday.
He was joined in his criticism of the superdelegate process on Saturday by fellow progressive stalwart Elizabeth Warren, who said at the Massachusetts Democratic State Convention that she "doesn't believe in superdelegates" -- even though as a senator, she is one.
"I don't think that superdelegates ought to sway the election," the Massachusetts senator said, MassLive.com reported
, an apparent reference to Warren's desire for superdelegates to not give the nomination to Sanders.
Warren has not endorsed a candidate in the Democratic primary fight. But behind the scenes, discussions between the Warren and Clinton camps have been markedly increasing
, especially as the freshman senator has begun to a play a more prominent role attacking Donald Trump, a source close to Warren has told CNN.