Blue Bell, PA. (AP) - Barack Obama says Hillary Rodham Clinton probably will win Today's Pennsylvania primary, but the contest may be closer than a lot of people think.

After days of sometimes bitter attacks, Obama tried to close out the campaign on a positive note, shying away from criticizing Clinton in his final appearances Monday night.

At one stop, Obama even mentioned a new Clinton ad that includes Osama bin Laden, calling that "a legitimate issue." The terrorist leader is seen along with images of the stock market crash, the bombing at Pearl Harbor, the collapse of the Berlin Wall and Hurricane Katrina.

Obama says his "number one task" if elected would be to keep Americans safe.

Clinton aides say they'll be grateful for a win in Pennsylvania, no matter how close.

Pennsylvania is the largest of the 10 contests remaining, with about 4 million registered Democrats and 158 delegates up for grabs in the primary.

(Copyright 2008 by The Associated Press. All Rights Reserved.)