's stinging defeat in West Virginia brings a sharp focus on the new coalition he may have to assemble to win the White House in November.
West Viginians rejected the presumptive Democratic nominee by a roughly two-to-one margin, one of the widest margins of the primary season. The outcome was the predictable result of familiar demographics: West Virginia's relatively poor white voters have been's base since February.
In a stark rejection of Obama in a state Bill Clinton carried in 1992 and 1996, almost half of the Democratic primary voters - typically the most partisan Democrats in a state - said they'd vote for Republicanrather than Obama in November.
The results also suggested a deeper dissatisfaction among the state's Democrats with both candidates: John Edwards, who dropped out more than three months ago, registered a substantial 7 percent of the vote, though Clinton immediately used the results to make her own case for electablity.
"I'm more determined than ever to carry on this campaign," Clinton told a cheering crowd in Charleston, stressing her electablity. "If you give me a chance, Democrats, I'll come back to West Virginia in the general election, and we'll win this state, and we'll win the White House."
Behind her, a young man waved a bowling pin - a symbol of the cultural distance between Obama, who bowls poorly, and the state's working-class white voters. Her supporters cheered raucously when numbers showing Clinton's margin among non-college educated voters flashed on the screen in Charleston.
But with Obama remaining as the likely nominee, the results highlighted the question of exactly how he will beat McCain in November, a question his campaign did not directly address in a memo released a few hours before polls closed.
The memo stressed Obama's strength in a different group, independent voters.
"Nationally, Obama is running stronger among Independent voters than any winning Presidential candidate since 1988 and is significantly outperforming Sen. Clinton among these voters as well in general election polling," the memo said.
The memo also dismissed as a "myth" the notion that "Obama cannot perform strongly enough among white voters."
"Obama ... is running as well or better than past Democratic candidates among white voters," the memo said, showing he currently holds a share of white support similar to that Al Gore and John Kerry held in their head-to-head contests in 2000 and 2004.
The results on which the campaign is relying indicate that Obama does somewhat better with educated white independent voters than Clinton, making up for his deficit with working-class white voters.
That's a demographic fact that could change the map in November, pushing Obama's campaign north and west, and posing problems for him in the crucial rust belt portions of Ohio and Pennsylvania.
Obama aides said Obama called Clinton, but didn't reach her, after her victory became clear.
But he spent the night in Missouri, and let the West Virginia results pass without comment.
By Ben Smith