HERSHEY, Pa. — On a recent Friday night, Pennsylvania Republicans tucked into a catered meal at the Hershey Lodge as Donald Trump Jr. took the stage, following his girlfriend, Kimberly Guilfoyle, in what has become their pattern on the campaign trail this fall.... ... He and Ms. Guilfoyle are instead radiating a carefree, tag-team vibe at events, in interviews and in a television ad for the super PAC supporting his father. Mr. Trump has echoed his father’s attacks on Democrats, mocked President Barack Obama for talking about a magic wand solving problems — “Abracadabra, buddy!” Mr. Trump chuckled from the...