Homicides and violent crimes have been surging this year in many cities, quite possibly because law enforcement has cut back on active policing techniques criticized by Black Lives Matter. But gun control is not necessarily voters’ preferred response. Homicides in Washington, D.C., have spiked 58 percent in 2015, and a Washington Post poll showed that concern about crime has spiked upward as well. But a ban on gun ownership is supported by only 51 percent of its residents, 91 percent of whom voted for Obama in 2012. Surprisingly, 47 percent oppose such a ban.

The current rash of campus rebellions, with students hurling epithets at liberal administrators and threatening violence against other students, could hurt Democrats as well, making promises of “free college” look less attractive.

It’s hard for a party to win a third consecutive presidential term. In the last 65 years, it has happened only once, when George H. W. Bush won in 1988. Richard Nixon and Al Gore came close to doing so in 1960 and 2000, but failed.