Ahead of last year’s midterm elections, I suggested to you on this page that Republicans had two years left to decide: Are they the party of Ronald Reagan, or the party of Donald Trump? A party of grounded conservative principles, advanced by Reagan’s high-minded optimism and penchant for compromise? Or a party of authoritarian, populist impulses, motivated by Trump’s dark and pessimistic vision of America as “a nation in decline”? For those of us who hoped that Republican voters would choose the former, recent months have offered little cause for optimism. Sen. Tim Scott’s withdrawal from the presidential primary earlier...