Once Obama was accepted by the black population, he was going to get a majority of their votes. What Clinton needed was to win the white votes by somewhat significant margins. That was actually possible as women are a significant majority (about 60%) of democratic primaries. All men, including white men started trending Obama's way, once Edwards was out. So she needed to win white women by significant margins and as a single subgroup, white women still forms the largest block of votes in democratic primary (35% in Virginia). Unfortunately for her she lost most of her edge in that category. She is going to get killed if she keeps splitting white women 53-47.
BTW, if you look at the exit poll, you will see that Obama actually won the white vote by 52-47, because of white independents, who voted for him 66-33.