Last week, we handicapped possible Democratic candidates for president. Now, the Republicans. There won’t be an incumbent president in the 2028 race, but there will likely be a sitting vice president in the running. That’s why any assessment of Republican presidential prospects begins and ends with JD Vance. And of course, his prospects begin and end with Donald Trump. The current president got him elected U.S. senator and then, two years later, picked him to be his running mate, elevating Vance to MAGA’s crown prince — and the nomination front-runner. Vance can learn a lot of lessons from other vice...