Mitch McConnell’s exit as Senate GOP leader this fall officially kicks off a succession battle that has so far played out only in the backrooms of the Capitol. The Kentucky Republican isn’t stepping aside until November‚ but three potential heirs have long loomed large in the Hill’s calculus. Somewhat confusingly‚ all three are white men named John: Minority Whip John Thune (R-S.D.)‚ former whip John Cornyn (R-Texas) and GOP Conference Chair John Barrasso (R-Wyo.). For the moment‚ very...