Joseph R. Biden Jr. didn’t just win the Michigan primary this week. The turnout and demographic patterns of voters, from African-Americans in Detroit to affluent suburbanites to working-class white voters in rural areas, provided evidence of a broader Democratic coalition than the party mobilized in 2016, a powerful warning shot to the Trump campaign.
In that last general election, Hillary Clinton struggled with white voters and independents, and underperformed with black voters compared with Barack Obama in 2012 and 2008. To the surprise of both political parties, Donald J. Trump carried the state by 10,700 votes, the first time a Republican had won Michigan in 28 years.
What stands out about Mr. Biden’s victory on Tuesday is that he performed well not only among his bedrock supporters, black voters, but also drew solid backing from other key demographic groups — including college-educated white women, moderates and those over 45 — in a primary that shattered Democratic turnout records by more than 30 percent.
“The massive high turnout is very, very bad news in Michigan for the Trump campaign,” said Richard Czuba, a nonpartisan pollster who has surveyed the state for more than three decades. “Everybody is motivated to vote. And in a state like Michigan, when you have a record turnout coming — and I think we do in November — that is a huge benefit to the Democratic nominee. There just aren’t enough Republicans in Michigan.”
Because Michigan was the first Obama-to-Trump state to vote in the presidential primary, an erstwhile brick in what Democrats once complacently called their Blue Wall, both parties pounced on Tuesday’s results for signposts pointing to November.
The Trump campaign saw weaknesses for a Biden candidacy in the general election, specifically his problem appealing to voters under 45. “The math works very well for the President,” said Tim Murtaugh, a spokesman for the Trump campaign.
Independent analysts like Mr. Czuba, however, said that while it was true that motivation to vote was elevated for both parties, high turnout in Michigan historically favors Democrats.
Mr. Trump’s surprise victory in Michigan was his narrowest win in the three Rust Belt states he pried from Democrats in 2016, a group that also included Wisconsin and Pennsylvania. Perhaps Democrats’ best hope for winning the White House — and Republicans’ best line of defense — is through these same states in the industrial Midwest.
Polling and the 2018 midterm trends suggest Michigan is the most likely of the three to swing back to Democrats this year. If Mr. Biden becomes the nominee, he will probably be especially well positioned to appeal to black voters, to suburbanites abandoning the Republican Party and to enough rural white voters to nick the president’s margins in what analysts call “outstate” Michigan.
“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.
Once a stronghold of blue-collar Democratic support, its voters became the original Reagan Democrats of the 1980s. Macomb County twice voted for Mr. Obama, but in 2016, as Mr. Trump skewered trade deals that he said hurt American manufacturing, Macomb swung dramatically again. Its working class and largely white residents preferred Mr. Trump by about 50,000 votes.
Ronna McDaniel, the chair of the Republican National Committee and a Michigander, predicted Macomb County would be the keystone in a second Trump victory in the state.
“Macomb is how we won Michigan,” she told Fox News on Tuesday. “Those Reagan Democrats became Trump Republicans and they have stayed with him since 2016. I’d say he’s even grown support in Macomb.”
She said the county’s white working-class workers would be “key to us winning Michigan again.”
The county lies north of Detroit, where Mr. Biden got into a heated argument on Election Day with a hard-hatted worker at a Fiat Chrysler plant who accused him of trying to “take away our guns.”
On Tuesday, Mr. Biden pulled 66,000 votes out of Macomb County, surpassing Mr. Sanders’s 44,000. Mr. Trump received 78,000 votes there in the Republican primary.
Representative Andy Levin, the freshman Democrat who represents Macomb County, predicted it would return to the Democratic column in November.
“Donald Trump now has a record, and his record is not growing good jobs in Macomb County,’’ he said, pointing to Mr. Trump’s opposition to raising the minimum wage and “a mini recession in manufacturing” in his district.
Federal data shows that Michigan manufacturing jobs increased annually since the recession through 2018, but have since dipped. General Motors last year closed a transmission plant in Macomb County, affecting 260 employees.
A former labor organizer, Mr. Levin predicted, “You’ll be back in Macomb writing the obituary for Donald Trump’s presidency after Nov. 3.”
That suggests a large-scale swing of Trump 2016 voters crossing over to the Democratic nominee. Given the loyalty of Mr. Trump’s base, it may be unrealistic.
But there is a second possibility, one suggested by Tuesday’s record turnout: that Democrats who did not vote in 2016 will come off the sidelines for their party’s nominee this year.
Michael Taylor, the Republican mayor of one of Macomb County’s largest cities, Sterling Heights, who voted for Mr. Trump, announced publicly days before the primary that he supported Mr. Biden.
While he doubts that there are many Trump voters who will defect, Mr. Taylor sees another path.
“What you could see, and what I think will probably happen, is that Joe Biden is going to attract some voters who sat out in 2016 because they didn’t like their options,” he said. “And you’re going to see a small percent who say, ‘I voted for Trump last time but I don’t think I can this time.’ That’s all it’s going to take in a swing state like Michigan.”