Clinton has consistently led in Pennsylvania, the largest prize remaining to the two candidates, but Obama has narrowed the gap.

The Illinois senator also has far more cash on hand, allowing him to open 30 offices in the state to Clinton's 21 and to spend five times as much on TV advertising.

But Mark Mellman told Politico Friday he does not think Obama is going to be able to cripple Clinton in Pennsylvania. He said that if he tries, he will allow Clinton to claim the state as a victory even if her margin is relatively small.

"Each of these two candidates has built a demographic coalition that, with the exception of Wisconsin, has stayed with them throughout. There's just not a lot of movement," Mellman said.

Pennsylvania is a strong state for Clinton because the Democratic base is heavy on women, seniors, Catholics and blue-collar workers, all groups that tend to go for the New York senator.