A six-week, increasingly hard-edged Pennsylvania primary contest between Democrats Hillary Rodham Clinton and Barack Obama culminated Tuesday as voters registered their choice - a decision that could save or sink Clinton's flagging candidacy. The candidates questioned each other's character and readiness to be commander in chief in last-minute television ads and barnstormed the state in a final pitch for votes in the most populous and delegate-rich state remaining in the nominating contest. Some 4 million Democrats were eligible to cast ballots, with 158 delegates at stake. Clinton was relying on a decisive win to reinvigorate her candidacy, while Obama hoped for an upset or a strong enough finish to secure the delegates needed to maintain his overall lead. Late polling showed Clinton with a single-digit lead in the state after besting Obama by 20 points or more in earlier surveys. As the polls opened at 7 a.m., the candidates engaged in a last round of sparring in pre-taped interviews aired on network and cable television. "What (Obama) has to demonstrate is to win a big state, a big state that Democrats need to win in order to achieve the presidency," Clinton told CBS' "The Early Show." "The road to Pennsylvania Avenue for a Democrat goes right through Pennsylvania." Obama, noting Clinton's polling lead, sought to lower expectations. "I think we've trimmed that back, but our view has always been that we're the underdogs here," Obama said. "I think she has to be heavily favored to win." Sarah Triplett arrived to vote Tuesday long before her suburban Philadelphia polling place opened and soon had plenty of company. "I had to be here to vote, and I pray that Obama does make a big change in a very positive way," said Triplett, a "65-plus" woman from Levittown who works with the disabled. In Allentown, where a line snaked out the door at First Presbyterian Church, 68-year-old Ellen Woolley, who works in finance, went for Clinton. Obama, she said, is a "marvelous speaker, but I really don't hear a lot of substance." The Illinois senator and his wife, Michelle, addressed a rally at the University Pittsburgh on Monday night. They were joined by Teresa Heinz Kerry, wife of 2004 Democratic nominee John Kerry. Heinz Kerry noted that her husband carried Pennsylvania in the general election. "I'm asking you to keep this streak going," she said. In Philadelphia, Clinton appeared with her husband, the former president, and their daughter, Chelsea, before a crowd at the University of Pennsylvania. "It's not enough to say 'Yes we can.' We have to say how we can," Clinton told the crowd, putting a twist on Obama's popular slogan of hope. The Pennsylvania contest turned sharply negative in its closing days as Obama cast doubts on his rival's honesty and trustworthiness. Clinton, in turn, questioned whether Obama was tough enough for the rigors of the Oval Office. The campaigns tangled Monday over a new Clinton television ad that invoked images of Osama bin Laden - the first time a Democratic candidate has used the mastermind of the Sept. 11, 2001, terrorist attacks in the 2008 race for the White House. "Harry Truman said it best, 'If you can't stand the heat, get out of the kitchen.' Who do you think has what it takes?" the ad says, as a picture of bin Laden and other national emergencies - from Hurricane Katrina to the fall of the Berlin Wall - flash on the screen. The Obama campaign moved quickly to counter the message, airing a response ad within hours that challenged Clinton's 2002 vote authorizing the U.S. invasion of Iraq. Obama addressed the matter himself at the Pittsburgh rally. "My job as commander in chief is to keep you safe. That will be my number one task," adding, "The war in Iraq was unwise." Clinton also was grilled about the ad by MSNBC's Keith Olbermann, one of a series of national television interviews the New York senator gave on Monday. "Since Senator Max Cleland was cut down by a commercial that featured a picture of bin Laden, that has been - that tactic has been kind of a bloody shirt for many Democrats," Olbermann said. He was referring to the former Georgia senator and disabled Vietnam War veteran who lost his seat in 2002 after Republicans questioned his willingness to fight terrorism. Clinton insisted the ad was about presidential leadership, not fearmongering. "There is nothing at all that is in any way inappropriate in saying, look, presidents face the unexpected all the time," she said. "If you were to hire the person you thought was ready on Day One to do the toughest job in the world, what would you look for? What kind of resume would you be trying to seek out?" Pennsylvania's demographics suit Clinton. The state has a higher median age, a higher percentage of whites, a lower median household income and fewer bachelor's degrees than the country overall. These are the voters - working-class whites and voters older than 50 - who have flocked to her in past contests. The Republican primary is also being held Tuesday, but Sen. John McCain of Arizona is already the certain nominee.