For half a century, Democrats owned Dubuque, Iowa, where blue-collar union workers built middle-class lives along the Mississippi River. Then, in 2016, Donald Trump became the first Republican to win Dubuque County since Dwight Eisenhower.

Two years later, those voters turned on Trump’s party, choosing Democrat Abby Finkenauer Abby Lea FinkenauerTrump asked Chamber of Commerce to reconsider Democratic endorsements: report House Democrats' campaign arm reserves .6M in ads in competitive districts GOP leader says he doesn't want Chamber's endorsement: 'They have sold out' MORE over Rep. Rod Blum (R).

In 2016, “people just wanted to shake things up. That’s their answer to it. Electing Trump was to shake things up,” said Mario Ruiz, the Iowa vice president of the Quad City Federation of Labor.

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Now, Ruiz said, those same voters are asking: “What’s he done for the labor people? He hasn’t done anything.”

Dubuque is one of 206 counties across the country that voted for Barack Obama Barack Hussein ObamaMichelle Obama and Jennifer Lopez exchange Ginsburg memories Pence defends Trump's 'obligation' to nominate new Supreme Court justice The militia menace MORE twice, and Trump in 2016. These so-called pivot counties are spread across the country, from timber country in the Pacific Northwest to the North Woods of Maine.

But they are particularly concentrated along the borders of Iowa, Minnesota, Illinois and Wisconsin, four states that are home to 75 pivot counties.

Taken together, those 75 counties illustrate the contours of a new political battleground, on which Democrats and Republicans are fighting to win white working-class voters who handed Trump the White House — and then handed Democrats the Speaker’s gavel two years later.

In the midterm elections, Democratic candidates for Congress won back 44 of the 75 pivot counties in the four Midwestern states.

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Nationwide, Democrats won 113 of the 206 pivot counties. In those counties, the Democrat improved on Clinton’s vote share in the 2016 election by an average of 21 percent.

Even in pivot counties that stuck with the Republican candidate, that candidate underperformed President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s 2016 margins by nearly 7 percentage points, according to an analysis by the nonpartisan website Ballotpedia, suggesting either that those voters have soured on Trump or that some of Trump’s core base decided not to show up if his name wasn’t on the ballot.

The midterm election results give Democrats a road map to win back the White House and illustrate the importance of those same regions to Trump’s own chances of a second term.

Both sides must appeal, party strategists said, to voters who backed both Trump and Finkenauer.

“We as candidates, or as members of Congress, can’t go coastal on people. Where we as Democrats have struggled is when there have been more of these coastal-type ideas that do not resonate in towns of 1,000 people,” said Rep. Cheri Bustos Cheryl (Cheri) Lea BustosThe Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally DCCC dropping million on voter education program Clark rolls out endorsements in assistant Speaker race MORE (D), who represents an Illinois district that borders Finkenauer’s.

In 2018, Bustos, who heads the Democratic Congressional Campaign Committee, won all nine of the counties in her district that backed Trump.

Some pivot counties flipped back to the Democratic column on the strength of tough incumbents like Bustos, or Reps. Ron Kind Ronald (Ron) James KindWisconsin Rep. Ron Kind wins primary Democrats exit briefing saying they fear elections under foreign threat Bottom line MORE (D-Wis.) and Collin Peterson Collin Clark PetersonThe Hill's Campaign Report: 19 years since 9/11 | Dem rival to Marjorie Taylor Greene drops out | Collin Peterson faces fight of his career | Court delivers blow to ex-felon voting rights in Florida Peterson faces fight of his career in deep-red Minnesota district Democrats for Life urge DNC to change party platform on abortion MORE (D-Minn.).

Others, like Rice County, Minn., Rensselaer County, N.Y., and Hidalgo County, N.M., gaveDemocratic candidates like Reps. Angie Craig (Minn.), Anthony Delgado (N.Y.) and Xochitl Torres Small (N.M.) the margins they needed to flip Republican-held seats.

And some of those counties that stayed with the GOP gave Republicans their only two pickups of the midterm cycle: Three of the four Minnesota counties on the Iowa border that backed both Obama and Trump helped elect Rep. Jim Hagedorn James Lee HagedornHispanic Caucus campaign arm endorses slate of non-Hispanic candidates Cook shifts 20 House districts toward Democrats Time to look at both sides of the medical ledger MORE (R), and two Iron Range counties helped send Rep. Pete Stauber Peter (Pete) Allen StauberTrump, Biden vie for Minnesota Minnesota Rep. Pete Stauber glides to victory in GOP primary OVERNIGHT ENERGY: EPA rule extends life of toxic coal ash ponds | Flint class action suit against Mich. officials can proceed, court rules | Senate Democrats introduce environmental justice bill MORE (R) to Congress.

The tumult in the four Midwestern states, where a total of eight House seats changed party control in 2018, shows the evolving geography of blue-collar swing politics.

For generations, the battle for the blue-collars was centered on the Ohio River Valley, stretching from Pennsylvania to Missouri’s border with Kentucky.

Where Democrats once held a majority of the 17 congressional districts along the Ohio River, today they hold just two, in Pittsburgh and Louisville, Ky.

“The Upper Mississippi Valley is ground zero in blue-collar American politics now. It is no longer the Ohio Valley, which has reliably flipped to Republican,” said Brad Todd, a Republican strategist and co-author of “The Great Revolt: Inside the Populist Coalition Reshaping American Politics.”

“From Quincy, Ill., all the way up to Duluth, on both sides of the river, every county is up for grabs.”

Those counties are crucial to both Democratic and Republican hopes of winning the 270 electoral votes necessary to take the White House in 2020.

Trump cannot afford to lose ground in the Upper Midwest, where he smashed through the Democratic blue wall to win Wisconsin and Michigan. If Democrats cannot rebuild that blue wall, their path to 270 narrows to longer-shot states like Arizona, Georgia and Texas.

That almost certainly means that Trump, and the eventual Democratic nominee, will make the industrial Midwest a cornerstone of their 2020 strategy.

“People who work hard every day, the folks who shower after work, the people who felt like they’ve been left out in the cold and nobody’s fighting for them, we’ve got to punch our way back into the hearts of working people. And we can’t just do that with words, we have to do that by showing up, by listening,” Bustos said.