PHILADELPHIA (AP) _ Democrats Hillary Rodham Clinton and Barack Obama questioned each other's character and readiness to be commander in chief in last-minute television ads as the six-week Pennsylvania primary campaign steamed toward an uncertain conclusion Tuesday.

The two Democratic hopefuls 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 save her flagging 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 polls opened at 7 a.m., both candidates were to appear on all the network TV morning shows Tuesday.

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."