I have also a hunch that Kennedy made a deal team Obama: "I'll get you the Catholic vote if you get socialized medicine done".
Know it in my bones.
Street credibility. Obama needed political support from poor urban blacks. Poor urban blacks distrust anyone from outside their community, eg, someone with less prominently black features, not born in an urban US environment, and having more education than they did.
Baraq is 50% white, was raised middle class in Hawaii, attended an elite college prep high school, went to Ivy League colleges, and was a law prof @ Univ of Chicago.
In order to gain the 99% support of the black underclass / black political establishment (which was the foundation of his political career in Illinois), he needed to do his time as a community organizer, marry a "real" black woman, and attend a "real" black church.