Trailing in the race for Democratic delegates, Clinton has been trying to portray Obama, a first-term senator, as too inexperienced to shoulder the heavy responsibilities and pressure as US president and commander-in-chief of the armed forces.
Obama's camp on Monday accused Clinton of trying to scare voters, after she launched television advertisement featuring images of Osama bin Laden, the al-Qaeda leader, as well as pictures of Pearl Harbor and the 2005 hurricane that swamped New Orleans.
Obama has refused to predict that he can win in Pennsylvania, but in an interview with a Pittsburgh radio station, said he would do well.
"I am not predicting a win. I am predicting it is going to be close and we are going to do a lot better than people expect," he told KDKA.
The Illinois senator is hoping for a landslide in the Philadelphia area, with its high numbers of African-American, young and more affluent voters, to counter Clinton's appeal in poorer, more conservative western parts of the state. Delegate count
Nine more contests are scheduled before the campaign concludes on June 3.
But after the 15-month race, neither Democrat is expected to reach the tally of 2,025 nominating delegates to claim the nomination outright.
Obama now leads by 166 pledged delegates, 1,417 to 1,251, and leads overall by 142 delegates, 1,652 to 1,510, according to a count by MSNBC.
Clinton leads among the so-called superdelegates, 259 to 235, MSNBC said.
Clinton needs to convince nearly 800 superdelegates - senior Democrat officials who vote at August's party nominating convention - that it would be too risky to pick the inexperienced Obama to fight John McCain, the presumptive Republican candidate, in November.