Howard Fineman has it all figured out:

But if Clinton continues to the next stage-if the results tonight allow her to fend off those telling her to quit—the next round is going to be a lot nastier. It’s going to get into Obama’s South Side Chicago roots; into some of the wilder statements of his longtime minister, Jeremiah Wright; and into the not-so-sly raising of doubts about Obama’s religious beliefs.  Read more