As much as it pains me to admit this, Andrew is exactly right.
Obama has 7 weeks to win Pennsylvania and it really is a must win for him. Not because he can't be the nominee without PA, but because decisive win may help push the Clintons to page two.
If this race goes to the convention, the Democrats will likely loose in November, because the nominee will then literally only have 6 weeks to make his/her case to the voters. McCain is making his case now and will be until November. All the hopes of this being a referendum for change in how the US is governed will be lost. There is no possible worse outcome.
Of course, the Clintons know this but don't care. They are perfectly willing to torch the party to get what they want, and a PA win for them will be just like Ohio and Texas. They will never go unless the chorus for them to exit is so strong they feel humiliated. The only way to even hope for such pressure, is a decisive Obama win in PA.