Pennsylvania Presidential Primary (27 images)

Democratic presidential candidate Sen. Barack Obama (D-IL) (L) waives after he received the endorsement of Sen. Bob Casey (D-PA) at a rally at Soldiers and Sailors Military Museum and Memorial in the Oakland section of Pittsburgh on March 28, 2008. (UPI Photo/Stephen Gross)

Democratic presidential candidate Sen. Hillary Clinton (D-NY) addresses students and supporters at a Solutions for the American Economy town hall meeting in the gym at the University of Pittsburgh at Greensburg in Greensburg, Pennsylvania on March 25, 2008. (UPI Photo/Archie Carpenter)

Democratic presidential candidate Sen. Barack Obama (D-IL) addresses supporters at Soldiers and Sailors Military Museum and Memorial in the Oakland section of Pittsburgh at the kick off his bus tour of Pennsylvania on March 28, 2008. (UPI Photo/Archie Carpenter)