Democratic presidential candidate Bernie SandersBernie SandersJimmy Carter: 'I hope there's an age limit' on presidency 2020 candidates keep fitness on track while on the trail Mark Mellman: The most important moment in history? MORE on Friday questioned whether presumptive GOP presidential nominee Donald TrumpDonald John TrumpJimmy Carter: 'I hope there's an age limit' on presidency White House fires DHS general counsel: report Trump to cap California trip with visit to the border MORE is a billionaire, as he often claims.

ADVERTISEMENT

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.