"I think there's a big burden on Sen. Obama tomorrow to prove that he can win a big state, because he hasn't really up until now," she said.
Obama appeared optimistic about his chances in Pennsylvania.
"I think all we do is just work as hard as we can," said Obama, when asked how he defines success in today's primary.
"Sen. Clinton started off with a big lead here. She had a 20-point lead," he said. "But we feel good about how we've chipped away at that lead."
Obama also took time to go after the GOP candidate John McCain and his recent remark that even McCain would be better than President Bush. But Obama said McCain would still mimic the current administration too much.
"Some of his instincts may be better than President Bush's, but the policies that he's putting forward right now essentially offer a continuation of George Bush's term," said Obama.
When asked whether McCain would be better than Bush, Obama responded, "He might be better in some areas, and he might be worse in others."
"Part of what we have to watch as this campaign unfolds is what is he willing to say to get elected," said Obama.
With the average price of gas in Pennsylvania hitting $3.49, Obama told ABC News that he would help voters fill their tanks by giving them more money.
"We've got to get some more money in their pockets because we're not going to immediately reduce oil prices on the world stage," said Obama. "But what we can do is provide a middle class tax cut that puts more money in the pockets of families to deal with higher costs."
In a lighter moment, Clinton was asked how voters should differentiate between the plans or "meal" she and Obama are "bringing to the table" for the American people.
"Well, I think I'm a more experienced cook, and I think my meal will be better seasoned," Clinton said laughing.