Last night’s split-screen image, of front-runner Donald Trump in his own venue and the non-Trump Republicans clustered together elsewhere, is the starkest representation yet of a party that is cleft in two. But there is something puzzling and ethereal about this schism. The opposing factions are not divided over a policy question, like pro– and anti–Vietnam War Democrats in 1968, or pro- and anti-slavery Whigs in the 1850s. Nor do they disagree on general ideology, as happened when conservative Republicans seized the party in 1964 from mainstream ones. The dispute between Trump and his antagonists does not center any disagreement so major or direct. It is a conflict between the party’s political methodology and its governing program.