Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.) is disappointed with former President Barack Obama Barack Hussein ObamaObama calls on Senate not to fill Ginsburg's vacancy until after election Senate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week MORE's $400,000 speech to a leading investment firm, calling it "unfortunate" that the Democratic leader would ally himself with Wall Street interests.

"I think at a time when people are so frustrated with the power of Wall Street and the big-money interests, I think it is unfortunate that President Obama is doing this," Sanders said on CBS's "This Morning."

"Wall Street has incredible power, and I would have hoped that the president would not have given a speech like this," he added.

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Sanders' comments echoed those of another vocal opponent of income inequality, Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.), who on Thursday said that she was "troubled" by the speaking fee the former president is reportedly receiving.

The New York Times on Wednesday reported that Obama would receive the equivalent of his annual salary as president to speak at Cantor Fitzgerald LP's healthcare conference in September. No official announcement has been made yet.

Still, the news has opened the former president up to criticism in recent days. But a spokesman for Obama rejected the idea that Wall Street would sway Obama, pointing to the financial reforms implemented during his time in office.

"With regard to this or any speech involving Wall Street sponsors, I'd just point out that in 2008, Barack Obama raised more money from Wall Street than any candidate in history — and still went on to successfully pass and implement the toughest reforms on Wall Street since [former President Franklin Delano Roosevelt]," Eric Schultz said in a statement.