Yglesias crunches the numbers here.
John Kerry won 41 percent of the (non-Hispanic — assume “white” = “non-Hispanic white” throughout this discussion) white vote, and Barack Obama improved to 43 percent of the white vote. But the white vote declined from 77 percent of the electorate to 74 percent of the electorate. As a result, 31.57 percent of voters were white people who voted for John Kerry in 2004. In 2008, the tally was very similar — 31.82 percent of voters were white people who voted for Barack Obama.
The big difference is that Obama increased the share of the black vote from 11 percent to 13 percent, increased the share of the “other” vote from 2 percent to 3 percent, grew his share of the black vote by seven percentage points, grew his share of the Hispanic vote by 13 (!) percentage points, grew his share of the Asian vote by five percentage points, and grew his share of the “other” vote by 11 percentage points. Consequently, while just 16.12 percent of 2004 voters were non-white for Kerry, fully 20.15 percent of 2008 voters were non-white for Obama. That 4.03 percentage point increase was the difference maker. McCain, despite the collapse in GOP party identification, despite Bush’s unpopularity, despite the economic crisis, held his own among white people. But he got slaughtered by a much bigger margin among non-whites, and his white base shrunk.