Democratic presidential candidate 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 on Friday questioned whether presumptive GOP presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE is a billionaire, as he often claims.

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During a rally in Santa Fe, N.M., on Friday, Sanders slammed Trump’s new alliance with casino magnate Sheldon Adelson.

“Donald Trump himself at least claims to be a billionaire,” he said. “We don’t know, but he probably is. He is getting help from a multibillionaire named Sheldon Adelson. So when you have multibillionaires supporting other billionaires to be elected president, that is not democracy; that is oligarchy.”

Adelson, a longtime major GOP donor, endorsed Trump last week.

Trump has drawn scrutiny for months for not releasing his tax returns, with many onlookers speculating that he may be hiding something about his financial past.