Democrats insist there’s no more urgent job than ensuring Donald Trump is a one-term president. Which is odd, given how hard they are simultaneously working to alienate the voters they most need to make that happen. See this week’s debate on gun control.

The weekend’s shootings in El Paso, Texas, and Dayton, Ohio, inspired the entire liberal-media complex to chant “gun control,” reflexively and predictably. A favorite demand is to expand background checks—never mind that these mass shooters, and those of recent years, bought...