BETHLEHEM, Pa. -- Hillary Rodham Clinton and Barack Obama are making their final arguments ahead of Tuesday's Pennsylvania primary.

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Clinton is running an ad that marks the first time a Democratic candidate has used Osama bin Laden in a campaign commercial in the 2008 race. The terrorist appears along with images from the stock market crash, the bombing at Pearl Harbor, the collapse of the Berlin Wall and Hurricane Katrina as an announcer tells voters the political contest is for "the most important job in the world." Obama's campaign is accusing Clinton of playing on voters' fears.

Obama tried to end on a positive note after days of escalating accusations against Clinton. He didn't criticize his rival as he spoke with voters in suburban Philadelphia.

Obama has already hinted that Clinton will win Tuesday's primary, but he said it will be closer than some people think. Preprimary polls show Clinton with a lead in the state she must win to sustain her candidacy.

Overall, Obama has 1,646 delegates to 1,508 for Clinton in the Associated Press' count, with 2,025 needed to clinch the nomination.

He also leads Clinton by 1,414-1,250 among delegates won in primaries and caucuses, while she has the advantage, 258-232, among superdelegates, the party officials who attend the convention by virtue of their positions.

More than 300 superdelegates remain publicly uncommitted and, in interviews, many told the AP they want a candidate who can capture the White House. At the same time, others said that before deciding which contender to support, they will give special weight to the candidate with the most delegates won in primaries or caucuses, or the one who won their state or congressional district.

Even Clinton's allies concede she must win the Pennsylvania primary, and some have suggested she needs a sizable victory if she is to have a chance of overtaking Obama. But as was the case in other states, a strong popular vote win would not necessarily translate into a major gain in delegates for the former first lady.

Democrats award delegates proportionally based on the vote in congressional districts, and some have more than others. A congressional district in Philadelphia with a large black population, for example, has nine delegates, more than any other. Numerous other districts have four delegates, and the likeliest outcome in most if not all is for a 2-2 split.

Pennsylvania's primary marks the end of a six-week intermission in the Democratic campaign season. Only seven other states, Guam and Puerto Rico will follow over the next six weeks.

Partners in a historic race, Obama and Clinton squabbled through a final weekend of campaigning.

In Bethlehem -- of all places -- Clinton said the Obama campaign had abandoned its pledge to remain positive and stick to issues.

"While my opponent says one thing and his campaign does another, you can count on me to tell you where I stand and you can count on me to tell you very specifically the solutions I'm offering for America," she said.

Obama seemed uninterested in backing down.

"I disagree with Senator Clinton that lobbyists are part of the system," he said. "I think they are part of the problem."

Obama said he hasn't taken money from federal lobbyists and scolded Clinton for suggesting he has.

"She's running ads now suggesting otherwise. In this campaign, I have not taken their money, and nobody can dispute that," Obama said.

The Illinois senator also was running a commercial critical of Clinton's health care proposals in what his aides said was a response to an ad aired by an independent group that supports the former first lady.

Also, Obama aired a commercial that said that every major newspaper in Pennsylvania had endorsed him. "The (Pittsburgh) Post-Gazette calls Hillary Clinton's attacks the 'cynical responses of old politics,"' it said.

Ironically, Clinton drew an endorsement from the Pittsburgh Tribune-Review, owned and published by Richard Mellon Scaife. The billionaire conservative personally funded many of the Republican investigations that plagued Bill Clinton while he was president.

McCain released a fundraising report that listed receipts of $15 million in March. It was less than Obama or Clinton, but his best monthly total of the campaign, and underscored his ability to tap donors who supported his rivals in the heat of the primaries.

Pennsylvania voters are probably not going to find out Tuesday which candidate won the most delegates to the Democratic Party's national convention.

As in other Democratic contests, Pennsylvania awards delegates based on the statewide vote and the vote in individual congressional districts.

But unlike most states, Pennsylvania has a large number of counties split into multiple congressional districts. That could delay results by a day or more as election officials work to assign votes to the appropriate districts.

Fifty-five will be awarded based on the statewide vote, which should be available Tuesday night. Another 103 delegates will be awarded based on the vote in individual congressional districts.

All delegates will be awarded proportionally, as they are in every Democratic contest.