The former president now controls the Republican Party by virtually every conceivable measure. He has a commanding lead in fund-raising and polling. His policies are a beacon to which most conservative lawmakers orient themselves in affairs both foreign and domestic. His endorsement remains the single most coveted asset that any Republican could hope to brandish in a primary race, and he has already received support from an overwhelming majority of prominent elected Republicans. In this election cycle, Mr. Trump has received endorsements from 130 House members and 31 senators, a majority of both Republican caucuses, according to an analysis by...