Clinton, favoured to win Tuesday's contest, needs a big victory margin to boost her chances of catching Obama in the Democratic race and to head off renewed calls to end her candidacy.

Joined by her husband, former President Bill Clinton, and daughter, Chelsea, the New York senator made a final appeal for votes at a rally in a packed arena in Philadelphia.

"I believe with all my heart that it is our moment, it is the time for the people of Pennsylvania to determine not just who the Democratic nominee will be, but who the president will be and what the future course of America will be," she said.

Clinton and Obama are duelling for the Democratic nomination to face Republican John McCain in November's presidential election. Both candidates spent the day scouring Pennsylvania in a late hunt for support.

Voting in the state ends at 8 p.m. EDT (1 a.m. British time) with first results available soon afterward.

Clinton launched a television ad stressing her ability to handle "the toughest job in the world" and featuring images of al Qaeda leader Osama bin Laden and damage from Hurricane Katrina.

"You need to be ready for anything -- especially now, with two wars, oil prices skyrocketing and an economy in crisis," the ad's narrator says, throwing in a reference to a famous saying by former Democratic President Harry Truman.

"If you can't stand the heat, get out of the kitchen," the narrator says. "Who do you think has what it takes?"

Clinton has questioned whether Obama, a first-term Illinois senator, has the experience to be commander in chief. Obama campaign spokesman Bill Burton noted the bin Laden imagery in the ad.

"It's ironic that she would borrow the president's tactics in her own campaign and invoke bin Laden to score political points," he said. "We already have a president who plays the politics of fear, and we don't need another."

After several days of sharp attacks on Clinton, Obama began the final day focused on pocketbook issues such as the cost of gasoline, taxes and jobs.

"We've had a terrific contest between myself and Senator Clinton and the other candidates who were originally involved," Obama told a forum with middle-class voters in the town of Blue Bell outside of Philadelphia.

"Democrats are pretty unified around some ideas," Obama said, citing the desire to provide universal health care and tackle global warming.

PLAYING DOWN EXPECTATIONS

Both camps tried to play down expectations in Pennsylvania, where Clinton's once double-digit lead has dwindled to single digits in many polls as Obama has outspent her heavily.

"I think it's going to be pretty close and we're campaigning hard," Obama said.

Obama told a rally in McKeesport, near Pittsburgh it was it was unfair for the media to question him about his patriotism as happened during an ABC news debate last week when he was asked whether he believed in the American flag and why he did not always wear a flag pin on his lapel.

"It frustrates me that people would even have a question about something like that because they don't ask the same questions of some of the other candidates," he said. "That concerns me."

Obama leads Clinton in delegates to the August convention in Denver, but neither can clinch the nomination without the help of superdelegates -- nearly 800 party insiders who are free to support either candidate.

Obama also picked up an endorsement from fiery filmmaker Michael Moore, who made he anti-Bush documentary "Fahrenheit 911."

"What we are witnessing is not just a candidate but a profound, massive public movement for change. My endorsement is more for Obama The Movement than it is for Obama the candidate," Moore wrote in a letter posted on his Web site.

Clinton hopes a big win in Pennsylvania and a strong run through the nine remaining Democratic contests will convince superdelegates she is the candidate who can capture the big states crucial to a November election victory.

Clinton has resisted calls from Obama supporters to pull out of the race and let him focus on the election battle against McCain.

McCain launched a five-day tour of economically struggling areas rarely visited by Republicans. He opened in Selma, Alabama, at a landmark of the U.S. civil rights movement -- the bridge where state police attacked more than 500 civil rights demonstrators in 1965 on a day known as "Bloody Sunday."

McCain's trip also will take him to the hard-hit steel town of Youngstown, Ohio, the Appalachia region of Kentucky and hurricane-stricken New Orleans.