Swann is seeking to become Pennsylvania's first black governor. Democratic Gov. Ed Rendell is running for a second term in November.
Swann had locked up more than enough unofficial support to win the endorsement of GOP leaders Saturday. The GOP primary is set for May 16.
Scranton, a wealthy businessman and former lieutenant governor whose father, Wiliam, was governor in the 1960s, said in a statement that he had doubts as to whether he could win over party leaders.
Scranton's campaign ran into trouble two weeks ago when he fired his campaign manager for saying Swann is "the rich white guy in this campaign." Scranton is white.