Record win is vital for Clinton

To overtake Barack Obama in the nationwide popular vote, Hillary Clinton needs a bigger win in Tuesday's Pennsylvania primary than she has had in any major contest so far. And that's just for starters.

After more than 40 Democratic primaries and caucuses, Obama leads Clinton by any measure:

Obama has won more states: 28, compared with 14 for Clinton.

He has accumulated more votes: 13.3 million, roughly 800,000 more than she has.

He has raised more money: $237 million, to her $193 million.

Most critically, Obama possesses more delegates to the party's national convention this summer in Denver. According to an unofficial tally by The Associated Press, Obama currently leads by a margin of 1,645 to 1,504 among pledged delegates and those superdelegates — elected and party officials who get an automatic vote on the nomination — who have indicated a preference. It will take 2,025 delegates to win the nomination.

Even if Clinton wins by more than 20 percentage points Tuesday — a landslide few experts expect — she would still have a hard time catching him.

Clinton needs "blowout numbers," says Peter Fenn, a Democratic consultant who isn't affiliated with either campaign. "The wheels would have to come off the Obama bus, and the engine would have to blow."

A popular-vote victory is vital to Clinton's chances because she is likely to end the primaries still trailing Obama in the race for delegates to the Democratic National Convention.

Two outcomes in the Pennsylvania primary on Tuesday could have an immediate impact.

If Clinton loses, aides expect that she would soon end a campaign that would be shorn of its rationale for continuing.

On the other hand, if Clinton exceeds the 10-point victory margin she posted in Ohio, she could make a significant dent in Obama's edge in the popular vote and reduce his delegate lead.

"I am a big believer that she needs either one, the popular or the delegate count," in order to make a case for why she should be the nominee, New Jersey Governor Jon Corzine, a Clinton backer, said in an interview.

Supporters say that winning more votes than Obama, 46, plus her primary victories in populous states such as California and Ohio, would prove she'd be the stronger candidate against Republican John McCain in the November election.