But the extent to which Mr. Trump is doing well among white working-class voters is important to Democrats. There are more white working-class voters than is generally believed, and Mr. Obama was stronger among these voters than typically assumed.

Mr. Trump’s speech on Tuesday was delivered in Monessen, Pa., the sort of place that typifies the overlooked Democratic resilience among white working-class voters, and that Mr. Trump needs in November. The area was once one of the most reliably Democratic parts of the country, and even today, Democrats still fare fairly well among white working-class voters. President Obama won Monessen by a 66-33 percent margin, and the town is at the tip of a region along the Monongahela River where Mr. Obama won among white working-class voters.

There are places like this — traditionally Democratic, white working-class areas where Mr. Obama still showed important strength — across the Northern United States. In Pennsylvania alone, there are similar regions along the Beaver River — north of Pittsburgh — or in the Scranton-Wilkes Barre area in northeast Pennsylvania. There’s Youngstown and Warren, Ohio — or a whole strip of towns along Lake Erie, from Toledo, Ohio, back to Erie, Pa. Many of these same areas broke heavily for Mr. Trump in the Republican primary.