Battleground Pennsylvania

How long the race goes on could well depend on Pennsylvania, a state that's been described as "Ohio on steroids," and is full of aging, white, working class voters, who have tended to favor Clinton.

The Clinton camp is arguing that electability comes from winning battleground states like Pennsylvania, where she currently holds a double-digit lead over Obama.

Obama is hoping an endorsement this week by the state's popular junior senator, Bob Casey, could help him win over some of those voters.