To make its case, the Sanders campaign is leaning on years-old criticism from Ms. Warren, the Massachusetts Democrat and liberal icon, over Mrs. Clinton’s vote in 2001 to make it harder for Americans to wipe out their debts through bankruptcy.

Mrs. Clinton, who has received millions from the financial sector in speaking fees and donations to her campaigns and charitable foundation, expressed regret over the legislation when she ran for president in 2008, though she had plenty of company among Democrats in voting for it.

And the Clinton campaign said the email from the Sanders camp was simply a veiled personal attack.

“If Senator Sanders means to allege that Secretary Clinton ever changed her position because of contributions, he should have the courage to say it, rather than continue to assassinate her character through insinuation,” Jesse Ferguson, a spokesman for Mrs. Clinton, said on Saturday.

Ms. Warren’s critique had its beginnings in 1998, when Congress was contemplating an overhaul to the bankruptcy system. Ms. Warren, a Harvard law professor, wrote an Op-Ed piece for The New York Times warning that under such an overhaul, a woman owed child support could lose some of her ability to collect it if the child’s father declared bankruptcy.