Barack Obama peaked in our national polling at a 55/38 spread last May. Now he's at 48/47. What's most interesting to note about that shift in his numbers is that it has come completely among white people. In May his approval with racial minorities was a 73/17 spread and now it's an almost identical 77/17. But with whites he's fallen from slightly positive ground at 48/45 to strongly negative territory at 37/58.
That is part of the problem for Democrats this year in the midterm election. While white voters were only 74% of the electorate in 2008, they made up 79% of it in 2006 the last time there wasn't a Presidential contest on the ballot. So before taking anything else into account the party is at a disadvantage simply because of the likely demographics of the electorate.
It's important not to jump to too many conclusions about 2012 based on what happens this fall though- just because a whiter electorate helps contribute to Republican victories this year doesn't mean all those nonwhite voters won't be back to the polls to reelect Obama in 2012. But Obama probably does need to get his numbers with whites turned around if he's going to get reelected.