Barack Obama said she would probably win but he hoped to keep it close in Tuesday's voting.

Clinton made her closing arguments Monday for the biggest primary left on the election schedule, running an ad with historic images to ask voters whom they would trust most in the White House during a time of trouble. It's the same tactic she used successfully in the "3 a.m." ad she aired in the closing days of the Ohio and Texas contests last month.

It was the first time a Democratic candidate has used bin Laden in a campaign commercial in the 2008 race for the White House. The terrorist appears along with images from the stock market crash, the bombing at Pearl Harbor, the Soviet threat, the collapse of the Berlin Wall and Hurricane Katrina as an announcer tells voters the political contest is for "the most important job in the world."

"You need to be ready for anything, especially now, with two wars, oil prices skyrocketing and an economy in crisis. 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 announcer says at the end, as Clinton appears on screen.

At his last event of the day, in Pittsburgh, Obama brought up Clinton's new ad and mentioned that it showed bin Laden.

"That's a legitimate issue," he said. "My job as commander in chief is to keep you safe. That will be my number one task." But he added that keeping the country safe, in part, means using the military wisely and said "the war in Iraq was unwise." He was trying to draw an unnamed contrast with Clinton, whom he long has assailed for voting to authorize the use of force in Iraq.

Earlier, Obama left the negative talk to aides, trying to end on a positive note after days of escalating accusations against Clinton. He didn't criticize his rival as he spoke with voters at an outdoor plaza in suburban Philadelphia, where tulips and daffodils bloomed and children played in the background.

"I thought this would be a nice change of pace," Obama said. A bank of TV cameras captured the soft images - early enough in the day to ensure that voters would see the images on Philadelphia newscasts the night before the primary.

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.

Obama suggested in an interview with Pittsburgh radio station KDKA that Clinton probably would come out on top. "I'm not predicting a win. I'm predicting it's going to be close and that we are going to do a lot better than people expect," he said.

Clinton aides tried to keep expectations down, insisting they would be grateful for a win no matter how close. While the New York senator began the race with a hefty 20-point lead in several polls in the state, Obama's extensive campaigning and heavy TV ad purchases significantly cut into her lead.

She told supporters in Scranton, "We really need to bear down in these last few days. The whole world is watching."

Clinton was hoping a big win in Pennsylvania could boost her chance at the nomination. A loss would increase pressure on her to exit the race, with Obama leading in delegates and the popular vote.

The primary comes after a six-week break in voting that allowed Clinton and Obama to spend more time in Pennsylvania than in any state since the primaries and caucuses began at the first of the year.

Pennsylvania is the largest of the 10 contests remaining, with about 4 million registered Democrats and 158 delegates up for grabs in the primary. Polls were to open at 7 a.m. and close at 8 p.m. EDT.

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Nedra Pickler reported from Washington. Associated Press writer Beth Fouhy in Scranton, Pa., contributed to this report.

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