The need for stability and experience should eliminate the young MAGA-in-a-hurry types like Sen. J.D. Vance or Members of the House. As the GOP convention nears, and voters focus on the election, the race remains closer than it should be given Mr. Biden’s infirmities. Swing voters still worry about Mr. Trump and the possibility of four more years of turmoil. The former President can help his campaign, and the country, by projecting stability and calm. We know many readers will think this is impossible, and maybe it is. Yet he showed admirable restraint after the debate last week, letting Democrats...