A campaign spokesman also confirmed that Obama had asked them to help the former first lady.
"Some of our donors have asked and Barack said if they have the ability to raise or give money to help on debt, we encourage them to do so," spokesman Robert Gibbs said.
Obama's directive came two days before he and Clinton meet in Washington on Thursday with some of her top fundraisers to get them on board with his campaign after their bruising contest for the Democratic presidential nomination.
Obama clinched the nomination earlier this month; Clinton suspended her campaign and endorsed him.
Clinton ended May $22 million in the red. More than half of the total was her own money that she poured into the campaign.
