WASHINGTON – For Barack Obama and Hillary Clinton, the question Tuesday in the Pennsylvania Democratic presidential primary isn’t so much who wins, but by how much.

The tiniest of wins for Obama would all but guarantee him the nomination, say pundits and pollsters. But with Clinton expected to win – she has led in almost every poll taken in the state – Pennsylvania’s Democratic primary has become more a referendum on whether she’s prolonging a losing battle or capable of staging a string of stunning victories that could lead to the party’s presidential nod.

Essentially, the thinking goes like this:

If Clinton 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 – after holding double-digit leads in polls a month or so ago and with the intervening weeks seeing Obama battle inflammatory remarks made by his former pastor and his own comments about some small-town Americans clinging to guns and religion because of their bitterness over government – it could make his nomination seem increasingly inevitable.

“What is a loss? That’s what the conversation is,” said Ann Selzer, president of Selzer & Co., a firm based in Des Moines, Iowa, that does polling for the Detroit Free Press. “If it’s 51 percent of the popular vote, is that enough?”

Maybe not.

A few weeks ago on NBC’s “Meet the Press,” Pennsylvania Gov. Ed Rendell – who has been an indefatigable asset for Clinton in the state – set the bar at 5 points, saying she’d win by at least that much.

Anything less may still score a victory – going along with those in New York, California, Texas and Ohio – without counting for much.

“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.

Even if she doesn’t win big Tuesday, few expect her to quit the race soon. For that to happen, Obama would need to beat her.

For a while it seemed that he would catch her in the polls, trimming her leads of a month ago to single digits in the last few weeks as he has reportedly outspent her campaign at least 2-1.

As of this week, the polls showed the race still close, but with Clinton having a clear advantage.

Rasmussen Reports had it Clinton 47 percent, Obama 44 percent in a survey with a margin of error of plus or minus 4 percentage points after last Wednesday’s debate, which was widely seen as a poor performance by Obama. A week earlier, Clinton had a 9-point lead in the poll.

Franklin & Marshall College, based in Lancaster, Pa., had it 46 percent-40 percent for Clinton in an April 8-13 survey that she led by 16 points in mid-March, and Quinnipiac University in Connecticut showed Clinton leading 50 percent-44 percent on April 9-13, after being up by 12 points in March.

The problem for Obama, said Clay Richards, assistant director of the Quinnipiac University Polling Institute, is that if the candidate’s remarks about small-town Americans haven’t caused people to turn from him, they have “certainly stalled his campaign.”

Neil Oxman, whose Philadelphia-based Campaign Group worked with Bill Richardson’s campaign and is now tracking political spending in the state, said he thought Obama was going to pull off an upset until the remarks about small-town Americans came out last weekend. Now, he’s less certain.

“The campaign was becoming a referendum on change. … I think he can still win based upon great turnout, but if you were a betting man, you’d say she’s going to pull the thing out by single digits,” he said.

He added: “No one here has any idea what the last few days have meant, and what they will mean on Tuesday.”

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 super delegates who can back whomever they want, but are widely expected to go to who is winning among pledged delegates in the states.

It also would allow her to strengthen her argument that while Obama has won in other parts of the nation, she has taken the biggest and most important battleground states – and that the only reason he is ahead is because of the way Democrats apportion votes in a win.

And don’t be surprised if news of a big Clinton win is accompanied by a new push to seat the delegates she won in Michigan and Florida who were disallowed because of the states’ early primaries, which violated party rules.

While Clinton may have undercut her own argument about Obama’s electability at Wednesday’s debate by acknowledging “yes, yes, yes” he could win the general election over presumptive Republican nominee John McCain, a large victory in Pennsylvania could serve as its own proof that Obama’s message is not resonating well enough among key Democratic voters.

His campaign has pumped about $9 million into Pennsylvania during the last six weeks. That may help explain why Obama plans to stay in Pennsylvania at least through tonight, instead of going to North Carolina, where he leads Clinton or Indiana, where the race is tight.

Those two states are next on the primary calendar, on May 6.

“She’s going to have a hard time catching him in the popular vote unless she has a huge win in Pennsylvania,” said David Bonior, the former Macomb County congressman who ran Democrat John Edwards’ unsuccessful campaign this year. As for Obama, Bonior said, “He can’t afford to have a huge loss.”

Even if she doesn’t win big in Pennsylvania, don’t expect Clinton to quit.

“The only thing that’s going to make her get out is that 50 super delegates they don’t expect stand up and say we’re endorsing him,” said Oxman. “If he wins by 1 vote, that will happen. If she wins by 6, no.”