PITTSBURGH — A double-digit victory by Senator Clinton in the primary here would extend the fight for the Democratic presidential nomination and breathe life into her campaign.

Polls going into today’s election included a Public Policy Polling survey showing Senator Obama leading by three points and two other polls, Suffolk University and InsiderAdvantage, suggesting Mrs. Clinton was ahead by 10 points. The overall trend for Mrs. Clinton in the state was upward. Prior to the Ohio contest, Suffolk came close to measuring Mrs. Clinton’s margin of victory, and it was one of the few pollsters to suggest a tight finish for her in New Hampshire, when most foresaw a sizable victory by Mr. Obama.

Speaking on the Pittsburgh radio station KDKA, Mr. Obama lowered expectations for his campaign. “I’m not predicting a win. I’m predicting it’s going to be close,” he said.

At stake today is the future of the Democratic nominating contest in which voters have been casting ballots for nearly five months, as well as the political fate of a New York senator who has been in the national limelight for 16 years. If Mr. Obama wins or loses only by a slight margin, money could dry up for Mrs. Clinton, a reality that could mean the end of her campaign. A sizeable victory for her, on the other hand, could propel her forward and represent the third major contest, after Texas and Ohio, during which Mr. Obama has failed to seal the deal with Democratic voters.

This primary, a Des Moines Register political columnist, David Yepsen, pointed out in an interview, is best thought of as analogous to the Iowa caucuses, where expectations mean more than the actual results.

Though Mrs. Clinton held a 20-point lead in some Pennsylvania polls six weeks ago, observers say she could certainly bill a 10-percentage-point win as a triumph, considering that Mr. Obama has spent millions more dollars on advertising in the state. “Perception-wise, if these polls hold up that would be a pretty decisive win with being outspent,” the director of Suffolk University’s political research center, David Paleologos, said. “I think it’s on to Indiana.”

A former chairman of the Democratic National Committee, Steven Grossman, who is a Clinton super-delegate, said that a big win would cut into Mr. Obama’s popular vote advantage and help with fundraising. “It keeps her legion of smaller contributors energized and continuing to fund her.”

Following an appearance by President Clinton in Millvale, the chairman of the Allegheny County Democrats and a supporter of Mrs. Clinton’s, James Burn, said she had received the support of two-thirds of participants in a straw poll. Pollsters expect Mrs. Clinton’s margin in the Western Pennsylvania to be high, but not that high.

Even if Ms. Clinton does win a convincing victory here in Pennsylvania, Mr. Obama possesses several advantages in a race for the nomination that is taking place on multiple levels. He will have won in twice as many states, hold the advantage in popular vote, reaped more delegates in primaries and caucuses, and gained in the fight for super-delegates.

“It is important to recognize that he is winning and she is losing,” the president of NDN, a progressive think tank and advocacy organization formerly known as the New Democrat Network, Simon Rosenberg, said. “If she doesn’t get a win that is convincing enough to replenish and reload her financial coffers, this race could be over quickly.”

A supporter of Mr. Obama who is president of New Future Communications, Jamal Simmons, said “if the race is closer than 10 points that will be a blow to the Clinton campaign and people will begin to call for her to lower the rhetoric, take down some of the negative ads and run on her issues, otherwise she will just start to look like a sore loser.”

“Anything short of her Ohio victory margin [10 points] will leave Clinton in a weak position to prevent a further and more accelerated erosion of support among super delegates,” a Brookings Institution senior fellow, Thomas Mann, said. “If Obama wins or loses narrowly, I suspect he will have a majority of delegates in two weeks, with his harvest in North Carolina and Indiana and a rush of support from uncommitted superdelegates.”

In a conference call Monday, Mrs. Clinton’s campaign team argued that even a small victory would be a boost for her campaign given Mr. Obama’s spending advantage in Pennsylvania. Said Mrs. Clinton’s campaign communication’s director, Howard Wolfson: “If he can’t win outspending us 3 to 1 in Pennsylvania and after having lost in Ohio, after outspending us 3 to 1, questions will again be raised about him winning those states he needs to win in November.”

A Democratic strategist who has contributed to both campaigns, Mary Anne Marsh, said Mr. Obama could be hurt both today and potentially in November by comments made by the Reverend Jeremiah Wright and by Mr. Obama’s own characterization of small-town and blue-collar voters as bitter. “People are kidding themselves if they think that these issues won’t be raised again and that people will ignore them,” she said. “Look at Michael Dukakis, Al Gore and John Kerry if you don’t think that people pay attention to these issues. This election isn’t going to be any different.”

Today’s election could help bring an end to what a senior fellow at the Brookings Institution, William Galston, called “a long slog.” But some polling data suggested the Democrats have incurred harm in the process. Mr. Paleologos said his polling indicated that as many as 20% of today’s voters would defect and vote for Senator McCain, the Republican, if their candidate is not the eventual presidential nominee.