Election 2014 displayed one of the most incredible switches of U.S. Senate control in recent memory. The Republicans did not just take back the upper chamber—they wiped the floor with the Democrats. They bested some of their pettiest political tricks, including three different Democratic candidates in Montana, plus three-way races in other states. Nothing can top Election Day 1980, which produced a Republican majority with 12 Senate wins. Election Day 2014 produced nine seats, but could have been 11 if not for the intense infighting in Republican primaries. So, will 2016 turn into an epic landslide of the Democrats and...