As the Democratic candidates began the finishing kick of their Pennsylvania marathon, a new poll depicted Sen. Hillary Rodham Clinton clinging to a slim lead over Sen. Barack Obama.

The poll showed Mrs. Clinton leading with the support of 48 percent of the Democrats surveyed compared with Mr. Obama's 43 percent. The five-point advantage was just outside the margin of error of the survey, conducted for the Post-Gazette, McClatchy Newspapers and MSNBC by Mason-Dixon Polling & Research.

The findings suggested that after six weeks of campaigning against a significantly better funded candidate, Mrs. Clinton retains the opportunity for a tactical victory.

With Mr. Obama holding a seemingly insurmountable lead in elected convention delegates, the broader question for Mrs. Clinton's underdog campaign is whether she can amass a winning margin that's big enough to propel her campaign through the final ten contests and on to the Democratic National Convention.

If Mr. Obama were to overcome her opinion-poll lead and win Tuesday, the long nomination battle would be all but over. Moreover, given the overall dynamics of the race, Mr. Obama can also win by losing -- provided that he does not lose by much.

A cliffhanger victory for Mrs. Clinton would make it difficult for her to press her last-ditch argument to the party's superdelegates that the Illinois senator is incapable of winning big states in the general election against Sen. John McCain.

In an MSNBC interview over the weekend, New Jersey Gov. Jon Corzine, a Clinton supporter, said a loss in Pennsylvania would be a "door-closer" for Mrs. Clinton's White House ambitions.

The Mason-Dixon survey showed many of the demographic contrasts noted again and again through the primary season. Mrs. Clinton did well with women, whites, older voters, Catholics and those most interested in the candidates' experience. Mr. Obama had an advantage among men, younger voters, Protestants, African-Americans and those interested in bringing change.

One respondent, Leonard Negvesky, 61, a laid-off factory worker from Lackawanna County, said he would vote for Mrs. Clinton because, "During her husband's administration we had surpluses. Since Bush came in, we have minus everything. I'd like to give his wife a chance and see what she can do. The economy is the biggest issue -- when you are 61, nobody wants to hire you -- they can't say it, but they don't want you."

Dr. Jim Snyder, 67, of Oakmont, a semiretired critical care doctor, said he favored Mr. Obama because of what he sees as his transformative brand of politics.

"He has generated an enthusiasm from people from all areas that we haven't seen in my lifetime, from kids in school to old folks. The place he is motivating us toward is comparable to where Clinton would have us go, but I think she is invested in a form of Washington politics that involves too many tradeoffs."

The Mason-Dixon interviews took place Thursday and Friday, after a tense and humorless debate in Philadelphia Wednesday night in which Mrs. Clinton apologized for her discredited account of a wartime landing in Bosnia, and Mr. Obama was pressed on his characterization of small town voters and on his associations with a former pastor and a neighbor who is a former member of the Weather Underground, a 60's radical group.

The tone of the debate was widely faulted. Critics cited it as an example of the news media's and the campaigns' preoccupation with the politics of "gotcha.'' But if there is some truth in that indictment, the debate and the overall campaign dialogue also reflected the fact that the Democratic campaign has turned from the beginning on questions of biography and character rather than fundamental disagreements on policy.

For the two remaining Democrats, as well as for those who fell by the electoral wayside earlier, there was relatively little disagreement on broad goals such as the need to end the war in Iraq and to move toward universal health care coverage.

Rather, they have contended over who was best equipped to achieve them. Mrs. Clinton argues that her experience, as first lady and as a second-term senator, has given her the insight to navigate Washington's bureaucratic and political thickets. Mr. Obama maintains that he offers an opportunity to break partisan logjams over such issue by transcending and transforming the partisan culture of Washington.

Wednesday's face-off at the National Constitution Center was the second Philadelphia debate of the Democratic campaign. A bigger cast of Democratic contenders had met at Drexel University last October, a time when the trajectory of the race was on a decidedly different path.

At that point, before the first primary or caucus vote had been cast, Mrs. Clinton appeared the clear favorite and most frequent target in the then crowded field. "They're not attacking me because I'm a woman," she said of her rival's criticism at yet another debate a few weeks later. "They're attacking me because I'm ahead."

Now, nationally, if not in Pennsylvania, it is Mr. Obama who is ahead and Mrs. Clinton on the attack -- although the Obama camp has shown its own sharp elbows in the waning days of the race.

The last ten days of the Pennsylvania campaign offered an increasingly sharp contrast in tone from its opening stages when both candidates concentrated on positive advertising and upbeat retail campaign appearances.

That changed after the Huffington Post We site reported Mr. Obama's remarks at a fundraiser earlier in the month in which he observed that voters in small towns in Pennsylvania and throughout the Midwest "cling to" institutions such as religion and hunting because they are "bitter" about their economic prospects. Mr. Obama apologized for what he described as mangled language in an attempt to make a point about voters' estrangement with traditional politics.

The Clinton campaign jumped on the remark like a linebacker on a fumble, not in the least because it diverted attention from the New York senator's own gaffe in having falsely claimed that she landed in Bosnia under sniper fire during her husband's administration. She faulted his remarks in numerous appearances, including the Philadelphia debate.

During the debate, Mr. Obama struck what seemed a conciliatory note by declining an opportunity to second the criticism of the Bosnia remark. His campaign aides have shown no such reluctance. Yesterday, they orchestrated a conference call of Bosnia veterans condemning the fanciful account and claiming that they undermined Mrs. Clinton's "moral authority" to be commander-in-chief.

One Clinton television ad continued to hammer Mr. Obama over his "bitter'' observations while the Obama campaign, in a new commercial, repeated its criticism of Mrs. Clinton's health care plan -- one that Mr. Obama has described as 95 percent similar to his own -- over a provision that would "force'' individuals to buy health insurance.

Mrs. Clinton's plan would mandate individual purchases of health coverage while promising subsidies for those who could not afford insurance, although the details of both the mandate mechanism and the promised subsidy remain vague. The Clinton campaign decried the ad in a conference call late Saturday, as Howard Wolfson, her communications director complained that, "He is running an negative campaign; he is throwing everything he can at us.''

The resources both sides are throwing at one another are unprecedented. On its own, Mrs. Clinton's Pennsylvania spending might well have set a record for a presidential primary in the state, except that the Obama campaign is outspending her by a margin variously estimated at between two and three to one.

Most of that has gone to television advertising, but the campaigns have also spent lavishly in areas such as mailings in which both candidates have assailed one another on trade.

The two Democrats have virtually identical positions on trade now, but their campaign dispute focuses on whether Mrs. Clinton did or did not privately oppose the North American Free Trade agreement when it was passed during her husband's administration. She claims she did and although supporters, such as AFSCME president Gerry McEntee back her up on that, other recollections of her position then suggest that insofar as she had reservations about the trade pact, they involved its political timing rather than its substance.

AFSCME is just one of the allied outside groups joining in the big bucks effort to persuade Pennsylvania voters. The feminist group NOW has also launched an ambitious effort on Mrs. Clinton's behalf including a major phone contact campaign. Among Mr. Obama's similar outside allies are the Teamsters and the SEIU.

The consensus of recent polling in the state is consistent with the new Post-Gazette survey in finding a race in which a Clinton lead once in double digits has narrowed. But there are outliers among the numerous outside surveys. At least one in the last ten days showed Mr. Obama in the lead while others describe a much larger Clinton lead. National polls betray similar variations. Gallup's latest daily tracking survey shows Mrs. Clinton in the lead for the first time in weeks while a Newsweek poll found Mr. Obama with a whopping 19-point advantage.

Someone's wrong.