“With the perception of a moderate ticket, the Dems are going to turn the industrial Midwest back blue,” said Greg McNeilly, a Republican consultant in Grand Rapids, who added that Mr. Biden’s pick for running mate would be critically important to his chances in Michigan.

In scores of rural counties on Tuesday, Mr. Biden beat Mr. Sanders by significantly larger totals than Mr. Sanders earned when he carried the same counties in the Democratic primary four years ago. The state’s rural enclaves are dominated by white voters without college degrees, the most loyal members of the Trump base. Even if most of them stick with the president, small gains by Democrats could make a difference in a closely fought statewide election.

“It suggests that Vice President Biden has a chance to stop the bleeding there,” said Brandon Dillon, a former chair of the Michigan Democratic Party.

Then there are the suburbs, the key to Democratic gains nationwide in 2018. Largely thanks to a swing by suburbanites in affluent Oakland County outside Detroit — the boyhood home of the 2012 Republican presidential nominee, Mitt Romney, whose father was governor of Michigan — Democrats rolled up large victories in 2018. They included the election of Gov. Gretchen Whitmer and two congresswomen who flipped Republican seats, Haley Stevens and Elissa Slotkin.

Turnout in the Democratic primary on Tuesday was up 40 percent in suburban counties, and 44 percent specifically in Oakland County, the second most populous county in the state after Detroit’s Wayne County.

“The districts that flipped from Republican to Democrat were largely located in large fast-growing suburban counties,” said Whit Ayres, a Republican pollster. “So Republicans are going to face the same challenge in 2020 that they faced in 2018 — how to be competitive in those larger suburban counties that contain large numbers of college educated women.”

Even if Republicans take a beating in Oakland County, just east of it is the state’s third-most populous county, Macomb, with its storied place in election lore.