(CNN) Former Democratic presidential candidate Bernie Sanders tried Monday to calm a crowd skeptical of Hillary Clinton, using a rally in advance of his convention speech to call for the party to unify against Republican presidential nominee Donald Trump.

"We have got to elect Hillary Clinton and Tim Kaine," Bernie Sanders said, which prompted some attendees to shout him down.

"Brothers and sisters, brothers and sisters, this is the real world that we live in," Sanders said, before adding, "Trump has made bigotry and hatred the cornerstone of his campaign. Throughout this campaign he has insulted Mexicans and Latinos. He has insulted Muslims. He has insulted women and African-Americans."

Sanders is expected to speak at the convention Monday night.

Earlier in his remarks on Monday afternoon, Sanders addressed the other elephant in the room: the impending resignation of Debbie Wasserman Schultz as chair of the Democratic National Committee.

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