Sen. Elizabeth Warren Elizabeth WarrenWarren, 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 No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.) said Thursday that she's "troubled" by the $400,000 fee that former President Barack Obama Barack Hussein ObamaThe Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon Trump appointees stymie recommendations to boost minority voting: report Obama's first presidential memoir, 'A Promised Land,' set for November release MORE will receive for a September speech to an investment firm.

"I was troubled by that," Warren said on SiriusXM's "Alter Family Politics" during an appearance to promote her new book.

"One of the things I talk about in the book is the influence of money. I describe it as a snake that slithers through Washington. And that it shows up in so many different ways here in Washington."

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The New York Times reported on Wednesday that Obama will receive the sum — equal to his annual pay as president — for a speech at Cantor Fitzgerald LP's healthcare conference, though there has been no public announcement yet.

Warren went on to describe the ills of money in politics more generally without specifically mentioning the president again. But she called on average Americans to fight back against the various ways that money can influence politicians and hurt the middle class.

"The influence of dollars on this place is what scares me. I think it ultimately threatens democracy," she said.

"And one of the reasons I feel so urgent when I wrote this book is that we have to think about what the tools are to fight back against it."

But in a statement about the speech issued before Warren's comments, Obama spokesman Eric Schultz pushed back against the idea that donations from Wall Street could influence Obama's politics.

"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]," Schultz wrote in the statement.