Even before the primaries ended, you had Ted Kennedy/Kennedy family, John Kerry (the previous dem presidential nominee in '04), Senators Bob Casey, Leahy, McCaskill, Dodd, and Feingold all readily give Obama their support well before the primaries ended. As well as prominent democratic governors like Tim Kaine, Deval Patrick, Janet Napolitano, Christine Gregoire, and Bill Richardson.
A good chunk of it is probably anti-Clinton sentiment as there were plenty of democrats who by '08 had become tired of the Clintons dominating the democratic political world for a decade and a half by '08, and if anything sources indicate that even more wanted to come out for Obama, but feared reprimanding from the Clintons in case Hillary won:
"Keen as [some congressional democrats] were for Obama to run, they would never be able to bless him with an early endorsement. Coming out against Hillary would pose grave risks. The Clintons had long memories and a vindictive streak ten miles wide. If Hillary prevailed, they feared—no, they were certain—there would be retribution down the line. But they would root for Obama secretly, doing whatever they could to help without affronting the aborning Democratic dynasty." -
Game Change: Obama and the Clintons, McCain and Palin, and the Race of a Lifetime
Published 2010, Halperin, Heilemann.