"I was troubled by that," the Massachusetts Democrat said Thursday on SiriusXM's "Alter Family Politics." "One of the things I talk about in the book ("This Fight Is Our Fight: The Battle to Save America's Middle Class") 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."
Warren, an icon of the left and influential Democratic leader in the Trump era, is the most prominent progressive to publicly question Obama's decision, particularly after Hillary Clinton alienated many liberals during the 2016 election over her coziness with Wall Street.
Despite the influence of the wealthy in politics, however, Warren said she is confident that the numbers are in favor of the American people.
"There's more of us than there is of them," Warren added. "And we've got to use our voices and our votes and fight back."
In the opening weeks of Obama's administration, the former president called bank CEOs "fat cats" amid widespread criticism about the golden parachutes given to execs whose companies contributed to the Great Recession.
But an Obama spokesperson dismissed the idea that the large speaking fee compromises the former president's convictions.
"As we announced months ago, President Obama will deliver speeches from time to time," Eric Schultz, a senior adviser to Obama, said Wednesday in a statement. "Some of those speeches will be paid, some will be unpaid, and regardless of venue or sponsor, President Obama will be true to his values, his vision, and his record."
"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 FDR," he added.
Schultz added that Obama will continue to focus most of his post-presidency on writing a book, giving speeches and "training and elevating a new generation of political leaders in America."