“She has been saying lately that she thinks that I am, quote unquote, not qualified to be president,” Mr Sanders told a crowd of more than 10,000 people at Temple University’s Liacouras Centre, Philadelphia.
“I don’t believe that she is qualified if she is, through her super PAC, taking tens of millions of dollars in specialinterest funds.”
Mr Sanders also said Ms Clinton is not qualified because of her vote on the war in Iraq and her support for trade agreements he says harm American workers.
It is the latest salvo in a war of words that has gotten more heated as underdog Mr Sanders gains ground on front-runner Ms Clinton, capped by the Vermont senator’s victory in Tuesday’s Wisconsin primary.
Ms Clinton’s spokesman, Brian Fallon, responded quickly to the comment, saying: “Hillary Clinton did not say Bernie Sanders was ‘not qualified.’ But he has now — absurdly — said it about her. This is a new low.”
Ms Clinton didn’t say Mr Sanders was “unqualified” or “not qualified” during a much-quoted interview on MSNBC’s Morning Joe.
In a discussion of an interview with Mr Sanders that appeared in the New York Daily News, Ms Clinton was asked if “Bernie Sanders is qualified and ready to be president of the United States”.
She said: “Well, I think he hadn’t done his homework and he’d been talking for more than a year about doing things that he obviously hadn’t really studied or understood, and that does raise a lot of questions.”
Mr Sanders’ spokesman, Michael Briggs, said Mr Sanders was responding to reports on the CNN and Washington Post websites.
A Post story was headlined: ‘Clinton questions whether Sanders is qualified to be president’.
Despite not called Mr Sanders unqualified, Ms Clinton has ratcheted up her attacks.
In a Politico interview, she said she tries to explain things in a more “open and truthful way than my opponent”.
Later, at a Philadelphia job training centre, Ms Clinton said people should know what she would do if she is elected, “not just lots of armwaving and hot rhetoric”.