CARROLL, Iowa—Democratic presidential contender Bernie Sanders, a fierce critic of the nation’s big banks, again singled out Goldman Sachs Group Inc. for its role in the financial crisis and its relationship with rival Hillary Clinton as particularly pernicious.

At a campaign event Tuesday, the Vermont senator railed against the company’s behavior, questioned why none of its executives have gone to jail, and suggested that the company exercises undue influence in Washington, contributing to a corrupt political system.

“Without naming any names, Goldman Sachs also provides very, very generous speaking fees to some unnamed candidates, very generous,” he said. “Now I know, I know that some of my opponents are very good speakers, very fine orators, very smart people, but you’ve got to be really, really, really good to get $225,000 a speech, that’s all I’ll say.”