Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE spoke with Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE in what he called a "gracious" conversation late Tuesday night, despite his promise to fight on in the presidential primary even as she's effectively clinched the Democratic nomination.

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"Tonight, I had a very gracious call from Secretary Clinton and congratulated her on her victories tonight," Sanders told the audience at a California rally.

"Our fight is to transform this country and to understand that we are in this together, and to understand that all of what we believe is what the majority of the American people believe."

The crowd loudly booed upon the mention of Clinton. Sanders put up both hands as to quiet the crowd, but did not directly address the response.

Neither side has publicly elaborated on the content of Tuesday’s conversation. But it's clear that Clinton wants to appeal to Sanders’s supporters, a theme she made a major piece of her victory speech Tuesday night after she clinched the majority of pledged delegates and superdelegates.

Clinton campaign chairman John Podesta teased that call earlier Tuesday evening on MSNBC, noting that party unity was a major concern and saying President Obama will likely step in to help bring the two sides together.

Obama called both candidates Tuesday night, the White House said, and Sanders will meet with the president on Thursday at the White House.

While Clinton has just about locked up the nomination, Sanders promised to "fight hard to win the primary in Washington, D.C.," next week and to "take our fight" to the July Democratic National Convention. It's unclear whether the "fight" he's referring to is a push to flip superdelegates and snatch the nomination away from Clinton, or a broader fight for the themes of his campaign to have a larger place in the party.