That struggle is playing out across the North, where Mr. Obama fared well among white voters four years ago. Ohio, the anchor of Mr. Obama’s so-called Midwestern firewall, remains very close. Mrs. Clinton has fared better in Wisconsin, but she’s not necessarily doing better there than she is nationwide.

The dynamic is also keeping many of the red, working-class states where Mr. Obama was competitive in 2008 — like Missouri, Montana and Indiana — out of the Democratic column.

Mrs. Clinton may yet sweep the Midwest, winning in places like Iowa and Ohio. But if she does prevail, she might do so in a very different way than Mr. Obama did four years ago.

She is making up for her weakness with strength in some of the most reliably Republican turf in the country. She’s running even with Mr. Trump in the Milwaukee suburbs; she leads in Western Michigan; and she’s posting huge leads in suburbs around Columbus, Ohio, and Philadelphia.

She’s struggling mightily in some traditionally Democratic or competitive areas like Green Bay, Wis.; northeastern Pennsylvania, including Scranton; northeastern Ohio, including Youngstown; and Macomb County, Mich. — the place that inspired the term “Reagan Democrats.”

This balance between Mrs. Clinton’s weakness among white working-class Northerners and her strength among well-educated voters might be enough to preserve a relatively similar electoral outcome in the Midwest, even as the underlying coalitions shift significantly. But this trade-off is not nearly as favorable for Mr. Trump in the states where there is much less room for him to make gains among white working-class voters.