Donald Trump is driving a wedge through the GOP over one of American politics’ thorniest issues: the future of Medicare and Social Security. The former president’s attacks on potential GOP primary opponents, and his warning to party leaders to stay away from the popular entitlement programs in their push to cut spending, are cleaving Republicans at every level. Lawmakers who once backed entitlement overhauls are now openly at odds with colleagues who’d prefer to soften their positions before they face voters in 2024. And a GOP presidential race that’s a referendum on Trump himself is now also becoming one on...