Pushing for a strong win to keep her White House hopes alive, Democrat Hillary Clinton touted her toughness on Monday on the eve of a showdown with presidential rival Barack Obama in Pennsylvania.
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.
