Democratic presidential hopeful, Sen. Barack Obama, D-Ill., left, looks at the crowd of supporters with Sen. Bob Casey at a rally outside the Lancaster, Pa., train station Saturday, April 19, 2008.
(AP Photo/Charles Rex Arbogast)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., right, shakes hands with Sen. Bob Casey Jr., D-Pa., as they arrive for a whistle stop in Lancaster, Pa., Saturday, April 19, 2008.
(AP Photo/Jae C. Hong)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., takes a drink of water after addressing a rally outside the Lancaster, Pa., train station Saturday, April 19, 2008. (AP Photo/Charles Rex
Arbogast)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., works the crowd at a rally outside the Lancaster, Pa., train station Saturday, April 19, 2008. (AP Photo/Charles Rex Arbogast)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., left, addresses supporters at a rally outside the Lancaster, Pa., train station with Sen. Bob Casey, Saturday, April 19, 2008.
(AP Photo/Charles Rex Arbogast)
Democratic presidential candidate Senator Barack Obama (D-IL) speaks to supporters during a rally at the Downingtown Station near Philadelphia, Pennsylvania, April 19, 2008. (Tim Shaffer/Reuters)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., walks to his campaign train after a whistle stop in Paoli, Pa., Saturday, April 19, 2008.
(AP Photo/Jae C. Hong)
Democratic presidential candidate Senator Barack Obama (D-IL) is joined by Pennsylvania U.S. Senator Bob Casey (R) as he speaks to supporters at the Wynnewood Station near Philadelphia, Pennsylvania, April 19, 2008. (Tim Shaffer/Reuters)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., listens as he is introduced by Sen. Bob Casey Jr., D-Pa., at a rally in Harrisburg, Pa., Saturday, April 19, 2008. (AP Photo/Jae C. Hong)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., campaigns at the State Capitol in Harrisburg, Pa., Saturday, April 19, 2008. (AP Photo/Jae C. Hong)
Democratic presidential hopeful, Sen. Barack Obama, D-Ill., shakes hands with supporter at a rally at the State Capitol in Harrisburg, Pa., Saturday, April 19, 2008. (AP Photo/Jae C. Hong)