PHILADELPHIA — This race will be different for Bob Casey. The three-term senator has faced challengers before, but never one as well-funded and with as much early GOP support as Dave McCormick. And the Pennsylvania Democrat made the stakes known at the first rally of his campaign in Philadelphia on Friday. “This is going to be a long, tough race,” Casey said from a podium at the Laborers’ District Council Training Institute on North Broad Street. “But I’m ready… because you know what, so much is on the line in this race for our commonwealth and for our country.” Casey, the son of a governor …