Obama's national finance chairman, Penny Pritzker, also donated $2,300, the maximum allowed under federal law, CNN reported.

"He wrote a check himself, as well as his finance chairman, so I got two checks in my pocket for Hillary," said Terry McAuliffe, who served as Clinton's chief campaign fundraiser.

Clinton, who suspended her campaign this month, has campaign debt of $22 million, including about $12 million the New York senator loaned to her own campaign.

Obama wrote Clinton the check the day before the two are to appear at a rally in Unity, N.H., where the two tied in the January primary.

Clinton has urged her supporters to back Obama. She said it would be a "grave" mistake for her supporters to cast their votes for presumptive Republican nominee, Arizona Sen. John McCain.

"If you care about the issues I care about, and the future that I outlined during my campaign, then you really have to stay with us in the Democratic Party and vote for Sen. Obama to be our next president," she said.