not sure I qualify as a thriend or insightful, but generally I'd think: 1) the more known a downballot candidate, the less benefit to campaigning with a presidential one 2) the closer the presidential race, the more risk to any downballot candidate... you want your own race in a toss-up it only makes sense to me if you're new or expect your president to win big; Casey has neither but his opponent is presumably new. but this is probably just a sign I don't know the standard wisdom here