US Democratic presidential candidate Senator Hillary Clinton (D-NY) addresses the crowd at a rally in Philadelphia, Pennsylvania, April 21, 2008.
Obama currently leads Clinton 1,648 delegates to 1,504, according to CNN's tally.
Neither candidate looks likely to pick up the 2,024 delegates needed to win the Democratic nomination before the August national convention, likely leaving some 800 Democratic superdelegates to decide who is the winner.
If Clinton loses in Pennsylvania, the biggest prize in the rest10 Democratic primaries, she will find it even harder to persuade those superdelegates to support her.
The former first lady acknowledged her White House bid is on the line Tuesday.
"Well, I have to win," Clinton told ABC. "I believe that's my task. And I'm going to do everything I can to win."
"I'm going to work as hard as I can between now and the times the polls close to encourage people to get out and vote and make sure that they're going to help pick the next president," she later told reporters during a stop in Conshohocken, Pennsylvania.
Size of victory
As latest polls all suggested Clinton is in a position to pull off the victory in Pennsylvania, the real question is not so much about whether she can win, but by how much the size of victory is.
If she wins big, with a double-digit margin, her argument that she's the best candidate gains steam and she fights on.
But if Clinton wins small, it could make Obama's nomination seem increasingly inevitable.
"She needs very significant wins in these remaining events in order to catch up," said Michael Traugott, a professor at the University of Michigan's Center for Political Studies.
Analysts said a big Clinton win could have several effects.
It could help cut Obama's lead in the popular vote -- which is seen as a path to attracting unpledged superdelegates who can back whomever they want, but are widely expected to go to who is winning among pledged delegates in the states.
"If she wins by a hefty margin, she can prove she can win the traditional Democratic constituencies needed to win the election, mostly white working-class voters," said Gloria Borger, a CNN senior political analyst.
"If Clinton wins by more than 10 points, which was her margin in neighboring Ohio and New Jersey, her campaign will have new momentum and she will soldier on," said Bill Schneider, another CNN senior political analyst.
"If she wins by single digits, we're in a political twilight zone. Nothing changes, "he said.
(Xinhua News Agency April 23, 2008)