Bernie Sanders has questioned whether his Democratic rival Hillary Clinton is "qualified" to be US president, after she spent much of the day criticising his record and his own preparedness for the job.
Mr Sanders added that Mrs Clinton is not qualified because of her vote on the war in Iraq and her support for trade agreements which he says are harmful to American workers.
Mr Sanders told a crowd of more than 10,000 people at Temple University's Liacouras Center in Philadelphia: "She has been saying lately that she thinks that I am, quote unquote, not qualified to be president.
"I don't believe that she is qualified if she is, through her super PAC (political action committee), taking tens of millions of dollars in special-interest funds."
This is the latest salvo in an increasingly heated war of words as underdog Mr Sanders gained ground on Democratic front-runner Mrs Clinton, capped by the Vermont senator's victory in Tuesday's Wisconsin primary.
Brian Fallon, a spokesman for Mrs Clinton, responded quickly to Mr Sanders' comment, writing on Twitter: "Hillary Clinton did not say Bernie Sanders was 'not qualified.' But he has now - absurdly - said it about her. This is a new low."
Indeed, Mrs Clinton did not say Sanders was "unqualified" or "not qualified" during a much-quoted TV interview on Wednesday morning.
In a discussion of an interview with Mr Sanders that appeared in the New York Daily News, Mrs Clinton was asked if "Bernie Sanders is qualified and ready to be president of the United States".
She responded: "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 he 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."
Whether or not Mrs Clinton said he was qualified or not, she has clearly ratcheted up her attacks on him. In an interview with Politico, she said she tries to explain things in a more "open and truthful way than my opponent".
Later, at a Philadelphia job training centre, Mrs Clinton said people should know what she would do if she's elected president, "not just lots of arm-waving and hot rhetoric".
On the Republican side, Donald Trump invoked the heroism of New York City police and firefighters during the 9/11 terror attacks as part of a swipe at his party rival Ted Cruz.
Mr Trump, in a rally on Long Island, addressed Mr Cruz's line from a debate earlier this year in which he criticised "New York values". Mr Trump said Cruz had made the remark "with scorn on his face" and "with hatred".
The Republican front-runner and New York native said he could not believe that anyone would question the heroism of the city's uniformed officers and construction workers during the aftermath of the 2001 attacks that toppled the World Trade Centre.