Sens. Barack Obama and Hillary Rodham Clinton showered each other with praise yesterday in their first joint appearance since the end of the Democratic presidential primary season at an event in which the senator from New York urged hundreds of her top donors to get behind the party's presumptive nominee.Clinton spoke first at the event at a Washington hotel, telling her disappointed supporters that Democrats "are a family, and we have an opportunity now to really demonstrate clearly we do know what's at stake, and we will do whatever it takes to win back this White House."Obama hailed his former rival and her backers. "I recognize that this room shared the same passion that a roomful of my supporters would show. I do not expect that passion to be transferred. Senator Clinton is unique, and your relationships with her are unique." But he added: "Senator Clinton and I at our core agree deeply that this country needs to change."He also sought to lead the move to unite by example, announcing that he had personally written a check for $2,300, the maximum he can give, to help retire Clinton's more than $20 million in campaign debt and that he had urged his biggest supporters to follow suit. The announcement drew a standing ovation. More than 200 Clinton fundraisers were on hand for the emotional event. Clinton supporter Terence R. McAuliffe, who emceed the event, said that the people in the sparse conference room had helped to bring in $230 million for her campaign.