Democrats may be hoping to gain a midterm wave. They may be hoping even more to boot Donald J. Trump from the White House in 2020. The president’s response to the Charlottesville violence, the rolling back of the Deferred Action of Childhood Arrivals, Russian collusion hysteria, the slow pace of enacting his agenda—all of these things have Democrats grinning, expect that they shouldn’t. The party’s own operatives and data crunchers have run through the numbers based on focus groups, and Democrats are still heading for the electoral ditch. They still have yet to mount an effective attack against the...