Casey, D-Pa., addressed an Obama rally in Pittsburgh, The New York Times reported. He had previously said he would endorse neither Obama nor Sen. Hillary Clinton, D-N.Y., to help bring the two senators back together once the race for the Democratic nomination is over.
"I believe in my heart that there is one person who's uniquely qualified to lead us in that new direction and that is Barack Obama." Casey said.
Pennsylvania, with 188 delegates to be selected in its April 22 primary, is the largest prize remaining. Clinton is expected to win but Casey's backing could help Obama, D-Ill., with Roman Catholic voters.
Casey's father, the late Robert Casey, served as governor from 1987-95. He was best-known outside the state for his opposition to abortion.