Mr. Santorum has insisted that voters do not really know much about Mr. Casey, that some Pennsylvanians actually believe the Democratic candidate is his father, former Gov. Robert P. Casey, who died six years ago.

"This is a guy recruited by the Democrats because he has a good name," Mr. Santorum said, after shaking hands at a Fourth of July fireworks display in Radnor.

Mr. Casey, in an interview, noted that he had run three statewide general election campaigns in recent years and served as state auditor general and treasurer. Moreover, he said, it is "really insulting to the people of Pennsylvania" to suggest that some voters still think he is his father.