Tomorrow could be a big step toward clearing the picture on the Republican side, or it may leave the waters muddied. Either way, the states remaining with late primaries are having unprecedented impact on the nomination process.

In the modern era of presidential campaigns (at least since 1980), most front-runners in both parties’ primaries usually wrap up the nomination in terms of delegates by Super Tuesday in late February or early March. This year, however, neither has yet been able to do so.In the Democratic primary, former Secretary of State Hillary Clinton is fairly close with 1,941 committed delegates so far against the 2,383 required to secure the nomination. 1,633 delegates are still to be decided, and she needs to win 27...