Update: From Larry Eichel of the Philadelphia Inquirer  

(Clinton's) 17-point loss in Wisconsin was a huge development, said Dante Scala, a political scientist from the University of New Hampshire.

Until Wisconsin, he said, Clinton had won primaries wherever her base of working-class women, seniors and Hispanics was inherently stronger than Obama's coalition of young people, blacks and upscale progressives.

"To use a tennis analogy, both candidates had been holding serve," Scala said during a talk at Villanova University last week. "Wisconsin was a breaking of serve." …

There's also the generational factor. "Maybe people don't want another baby boomer, another Clinton or Bush," said Doug Hansmann, a Clinton supporter from Madison, Wis. "The irony is that of the two Clintons and two Bushes, I think she's the best of the bunch."