In Pennsylvania, it's not just about winning for Hillary Clinton. It's about winning big.

Clinton trails Barack Obama in delegates and is struggling to convince uncommitted superdelegates that her campaign for the Democratic presidential nomination is still alive and kicking. To that end, her campaign is looking for a commanding victory Tuesday to give her momentum going into the Indiana and North Carolina primaries two weeks later.

"A double-digit victory in Pennsylvania would be huge," Clinton supporter and former Pennsylvania Rep. Joe Hoeffel told FOX News. "That double-digit victory is within reach and it would be a tremendous turnaround."

With three days to go until the Pennsylvania primary, both candidates stormed through the Keystone State on Saturday, taking jabs at each other's character. Obama told voters Clinton is a "slash and burn" Washington game-player. Clinton suggested Obama is all talk, no substance.

Polls show Clinton consistently leading in Pennsylvania, but since late March, Obama has narrowed that margin to about 5 or 6 percentage points.

Obama aides tell FOX News the goal is to keep Clinton's lead to single digits. This would dull Clinton's argument that she consistently wins big in large industrial states that are key in a general election. Clinton won by 10 points in Ohio and New Jersey and by 17 in her home state of New York.

In a sweep across the southeastern part of the state, Obama clambered aboard a shiny, royal blue train car Saturday morning in Philadelphia after speaking to about 35,000 supporters the night before -- the largest crowd of his campaign.

Clinton, meanwhile, spoke under a baking sun outside West Chester's 175-year-old fire house, striking a somber note about problems at home and abroad as she described the stakes for voters Tuesday.

"I don't want to just show up and give one of those whoop-dee-do speeches and get everybody whipped up," she said. "I want everyone thinking."

The primary Tuesday follows a month-long hiatus in voting, a gap the candidates filled in large measure by sullying each other.

Clinton has been trying to cast Obama as a flimsy candidate, even poking fun at him for complaining about the rough treatment he got at a debate Wednesday in Philadelphia. On Saturday she said the "incredible pressures of a political campaign" are a good way to test which candidate can handle the pressure of the presidency.

Clinton worked hard over the past week, in interviews and in a television ad, to get electoral mileage out of Obama's controversial claim to a group of California donors that small-town voters "cling" to guns and religion out of bitterness over lost jobs.

But only Tuesday will tell how well that worked. Pennsylvania polls have stayed pretty much steady for the past week while national polls have fluctuated wildly.

The Gallup daily tracking poll Saturday showed Clinton with the national lead, by 1 percentage point, for the first time since mid-March. The poll gave her 46 percent to Obama's 45 percent.

But a Newsweek poll released Friday gave Obama a 19-point lead, his widest ever of the campaign. The Gallup poll Friday gave Obama a 3-point lead.

Pennsylvania political consultant Ken Smukler told FOX News that the voter makeup in Pennsylvania favors Clinton. And her effort to play up Obama's "bitter" comments could pay a dividend, however small, he said.

"I think it's enough to move Hillary Clinton from high single digits to maybe low double digits, which is a significant move," he said.

Smukler said a few percentage points could make the difference in how undecided superdelegates view Clinton's candidacy. And Clinton needs superdelegates in order to overcome Obama's lead. Those unbound party officials continued drifting toward Obama since the last election, increasing his edge in the race, and that trend is bound to accelerate if he performs strongly Tuesday.

With 158 pledged delegates, Pennsylvania also offers the biggest payoff of any remaining primary state and a key opportunity for Clinton to make up ground against Obama's delegate lead.

Working in Clinton's favor, Pennsylvania has a large white, working-class population, and the third oldest population of any state. The state's politically savvy governor, Ed Rendell, backs Clinton.

But Obama is campaigning with Pennsylvania Sen. Bob Casey. The Obama campaign has 30 field offices to Clinton's 24 and has outspent her more than 2-to-1 on TV advertising.

Obama disputes that he's been complaining about the debate. And he also played on poll findings indicating unease with Clinton's veracity.

"She's taken different positions at different times on issues as fundamental as trade, or even the war, to suit the politics of the moment," Obama told the crowd Saturday in Wynnewood, Pa. "And when she gets caught at it, the notion is, well, you know what, that's just politics. That's how it works in Washington. You can say one thing here and say another thing there."

He amplified the point at a later stop, in Paoli.

"Senator Clinton's essential argument in this campaign is you can't change how the game is played in Washington. Her basic argument is that the slash-and-burn, say-anything, do-anything special interest-driven politics is how it works. ... Senator Clinton has internalized a lot of the strategies, the tactics, that have made Washington such a miserable place."

At his massive rally in Philadelphia the night before, he also played up his humble political beginnings, saying "we're still the underdog here in Pennsylvania."

Clinton's campaign is taking care not to heighten expectations for Pennsylvania.

"This is not going to be a blowout race. ... We're looking for a win, and we think it's going to be a close race," said Nick Clemons, her Pennsylvania director.

But her surrogates are already talking about how important a big win would be for her.

Former President Clinton told a small crowd in Wilkes-Barre: "You give her a big vote out of here, she'll wake up in a different world, and so will you, and America will have a better tomorrow."

One of Clinton's supporters, New Jersey Gov. Jon Corzine, said Friday that Clinton needs a big win in Pennsylvania if she hopes to overtake Obama, saying a loss in the Keystone State would be "pretty much a door-closer."

Obama's campaign released a statement Saturday saying Clinton shouldn't kid herself: "Senator Clinton's campaign has resorted to denying both the obvious - and their previous comments - by saying they'd be pleased with a narrow victory in Pennsylvania on Tuesday ... Senator Clinton needs a blow-out victory on Tuesday to meet expectations."

Obama leads Clinton in overall delegates, 1,645-1,505, with 2,025 needed to win the nomination. Obama also has a thin lead in the popular vote that Clinton hopes to overcome before the final ballots are cast in June.

FOX News' Major Garrett and Steve Brown and The Associated Press contributed to this report.