(CNN) -- Sen. Barack Obama has asked top contributors to help his former rival for the Democratic presidential nomination, Sen. Hillary Clinton, retire her debt, an Obama campaign source said Tuesday.

Obama did not direct members of his National Finance Committee to contribute to Clinton's campaign, the source said, but asked them to do so if they were so inclined.

Clinton suspended her campaign and endorsed Obama for the Democratic presidential nomination this month.

She has amassed a campaign debt of about $22 million, but about $12 million of that is money the New York senator loaned to the campaign herself.

Individual donors can contribute $2,300 to individual candidates.

Clinton and Obama have scheduled a joint campaign appearance Friday in Unity, New Hampshire.

Obama said Tuesday that he had spoken with Clinton by phone earlier in the day as well as on Sunday.

"We had a good conversation," he said. "We're looking forward to seeing each other tomorrow and campaigning on Friday."

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