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.

"Popular vote matters," says Steve Grossman, a marketing executive and one of Clinton's top fundraisers. "If there is an opportunity for her to pick up enough popular votes, that is a powerful calling card to the superdelegates to say the will of the people is a split decision."

To earn that split decision, though, Clinton would need a 25-point victory in Pennsylvania, plus 20-point wins in later contests in West Virginia, Kentucky and Puerto Rico. Even that scenario assumes Clinton, 60, would break even in Indiana, North Carolina, South Dakota, Montana and Oregon — a prospect that's not at all certain.

More than just big margins, Clinton would need record voter turnout too. In Pennsylvania, she would need a turnout of 2 million, about half the state's registered Democrats; in the 2004 primary, about 800,000 voted. She would also need turnout to almost double in other states where she leads, and reach some 1 million in Puerto Rico, which is about how many Democratic-leaning voters went to the polls in a 2004 gubernatorial election. The territory, known for its high turnout, didn't have a presidential primary that year.

In Pennsylvania — where a Bloomberg/Los Angeles Times poll gave her just a five-point margin last week — Clinton would need to win a strong majority of the state's suburban voters, about half of male voters, three-quarters of the rural vote and probably 70 percent of white voters, says Chris Borick, director of the Muhlenberg College Institute of Public Opinion in Allentown. She would also have to erode Obama's strength among black voters and college students.

"The analogy I would put out there is she has to have a near-perfect game in baseball," says Borick. "If she squeaks out a couple-point win, the math goes from bad to awful."

To shrink Obama's 800,000 popular-vote margin, the Clinton campaign argues for the inclusion of votes cast in Michigan and Florida. Those two states lost their right to send delegates to the convention by scheduling their contests earlier in the year than party rules allowed.

Clinton and Obama agreed not to campaign in the two states, and Obama took his name off the ballot in Michigan. Clinton won both uncontested races, and now says they should count in the nationwide popular-vote calculations.

There's almost no chance that party officials will give credence to those results. "No one is going to buy the argument that you have to count Michigan and Florida," says Allan Lichtman, a professor of political history at American University in Washington.

Instead, Clinton's slim prospects may rest on persuading enough of the 795 superdelegates that she has the better chance of defeating McCain. The superdelegates "first and foremost vote for the candidate they think is ready to be president and win in November," says Doug Hattaway, a Clinton campaign adviser.

Polls on the general election don't support the case that Clinton would make the stronger national candidate; they show little difference in head-to-head match-ups between McCain, the 71-year-old Arizona senator and presumptive Republican nominee, and either Clinton or Obama.

According to the average of national surveys compiled by, McCain leads Clinton by 46 percent to 45 percent, and is tied with Obama at 45 percent. The results are within the margin of error.

Obama "hasn't stumbled in a big way that makes him look unelectable in the fall," says Michael McDonald, a political scientist at George Mason University in Fairfax, Virginia.

Still, says Fenn, Clinton will be looking to make the "momentum argument" if she can pull off victories in most of the remaining primaries — arguing that would prove that hers is the campaign that is now "clicking on all cylinders."

That argument, he adds, is "a hard one to make if you don't have the popular vote and you don't have the delegate count."

Contributing: Jonathan Salant and Adam Cataldo, Bloomberg News; John Harwood, New York Times News Service