Hillary Clinton has built a commanding lead over Donald J. Trump in national polls, but she still has one big weakness: white working-class voters, especially men.

Even now, she is underperforming any recent Democratic candidate among white voters without a college degree.

It’s a very different story from 2008, when Barack Obama built a big national lead by attracting white working-class voters in states like Wisconsin and Indiana.

Instead, Mrs. Clinton’s gains come from big margins among well-educated voters and an electorate that’s much more diverse than it was even a decade ago.