Hillary Clinton is seeking a convincing win to keep her White House hopes alive today as Pennsylvania Democrats choose between her and Barack Obama in their presidential nominating contest.
Polls close at 8 pm EDT (0000 GMT) with results expected shortly after.
The New York senator is favored in Pennsylvania but needs a substantial victory to gain ground on Obama in the Democratic race and convince party leaders she is the best candidate to face Republican John McCain in November's presidential election.
Clinton's one-time 20-point lead in the state has slipped to single digits in many polls amid an onslaught of advertisements by Obama, who has heavily outspent her in the first nominating contest in six weeks.
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.
Former president Bill Clinton joins his wife Senator Hillary Rodham Clinton and daughter Chelsea at a campaign rally in Philadelphia last night
"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.
Mrs Clinton and Mr Obama are vying 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.
The latest poll showed Mrs Clinton leading by seven points, down from 12 points in mid-March.
Mrs 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?"
Mrs Clinton has questioned whether Mr 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 Mrs Clinton, Mr Obama began the final day focused on economic issues such as the cost of fuel, taxes and jobs.
"We've had a terrific contest between myself and Senator Clinton and the other candidates who were originally involved," Mr Obama told a forum with middle-class voters in the town of Blue Bell outside of Philadelphia.
"Democrats are pretty unified around some ideas," he said, citing the desire to provide universal health care and tackle global warming.
Mr Obama holds a near-insurmountable lead over Mrs 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. He is ahead by 700,000 in the accumulated popular vote and senior party figures have been moving steadily in his direction.
Mrs 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.
She has resisted calls from Obama supporters to pull out of the race and let him focus on the election battle against Mr McCain.
Mr Obama also enjoys an overwhelming financial advantage, with more than $40 million cash-in-hand at the start of April, while Mrs Clinton had $9 million - and $10 million in debts.
Mr McCain is on a five-day tour of economically struggling areas rarely visited by Republicans. He opened in Selma, Alabama, at a landmark of the US civil rights movement. Mr 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.