Sen. 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 (I-Vt.) fired back at Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE after the brash GOP presidential nominee said Sanders "sold out" and abandoned his supporters.

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Trump has knocked Sanders since he endorsed Clinton. He took aim at the Vermont senator while he was speaking in Philadelphia, saying he "totally sold out to Crooked Hillary Clinton."

"All of that work, energy and money, and nothing to show for it! Waste of time," he added.

Bernie Sanders totally sold out to Crooked Hillary Clinton. All of that work, energy and money, and nothing to show for it! Waste of time. — Donald J. Trump (@realDonaldTrump) July 26, 2016

Sad to watch Bernie Sanders abandon his revolution. We welcome all voters who want to fix our rigged system and bring back our jobs. — Donald J. Trump (@realDonaldTrump) July 26, 2016

Trump has tried to play up frustration among progressive voters as Democrats try to project unity at their four-day event. He also said Monday that former Sanders backers were welcome to join his campaign.