In the Cleveland debate, Donald Trump refused to commit to support whomever the Republican Party nominates in 2016. Trump would be wise to maintain his freedom of action. For there is a plot afoot in the Washington Post Conservative Club to purge Trump from the Republican Party before the primaries begin. “A political party has a right to … secure its borders,” asserts the Post’s George Will, “a duty to exclude interlopers.” Will wants The Donald “excommunicated” and locked out of all GOP debates until he kneels and takes a loyalty oath to the nominee. “Marginalizing Trump” carries no risk...