Around 40 percent of white Democrats without a college degree agree that free trade has done more harm than good (exit polls showed the same thing in the Democratic primary), or think the United States should get tougher with China on economic issues. Similar numbers agree with Mr. Trump on guns and immigration. All of Mr. Trump’s stances are more popular than traditional Republican views on the welfare state or culture war issues like abortion or same-sex marriage.

The potential for Mr. Trump to break through among white working-class voters isn’t merely theoretical. Recent public opinion surveys — even those showing Mr. Trump trailing Mrs. Clinton by a wide margin — all show him leading Mrs. Clinton by a wider margin among white voters without a college degree than the margin by which Mitt Romney led President Obama in 2012. Over all, Mr. Trump leads among white voters without a college degree by a 57-to-31 margin in the last six national polls. Mr. Romney led by a 55-to-37 percent margin with those voters in a compilation of the final polls in 2012.

These gains have not been enough to give Mr. Trump an overall lead. He trails by an average of five points in the same surveys.