Mr. Trump has a narrow path to the presidency: run up the score among white working-class voters so that he can overwhelm losses among well-educated and nonwhite voters. So far, polls show, he isn’t pulling it off. His gains among working-class white voters have come at an equal or greater cost among other voters.

This makes gun control a potentially useful issue for Mr. Trump. It’s something he can stress to white working-class Democrats without suffering too much attrition among traditional Republican voters.

Mr. Trump’s proposal to bar Muslim immigration might be riskier. Back in 2014, only a quarter of white working-class Democrats agreed that Islam was likelier to encourage violence, and only slightly more than half of Republican-leaners agreed. More recent surveys about Mr. Trump’s proposal to bar Muslims from entering the United States show a split over whether a majority of Americans support his position.