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DUBUQUE, Iowa - One of the essential storylines of the 2016 presidential election was the hidden Donald Trump voter: the person who wasn't surveyed by pollsters or comfortable telling friends or family about who they thought was best to lead the country.

Three years ago, thousands of these Americans - many working class, residing in the middle of the country - helped deliver the most astounding electoral surprise in modern history. Now, as they review the Trump presidency a year before his reelection, some are showing signs of turning on conventional wisdom again.

Heather, a local administrative assistant here who is married with a stroller-bound toddler, is still so embarrassed by her 2016 vote that she wasn't comfortable revealing her last name.

"I'm ashamed to admit ... but I'm of a more conservative bent and family. I just couldn't vote for Hillary. But I also thought there's no way Trump was going to win. This way, I could at least say, 'Well I didn't vote for her,'" she said. "I didn't think I was doing any harm."

While Heather isn't sharing her conversion far and wide, she's already decided "there's no chance" she'd vote for Trump again. "Heavens no," she said. "Trump basically turned me into a Democrat."

The 2020 presidential campaign has been engrossed in a debate over which demographic groups Democrats should devote most of their attention to in order to reclaim the White House. African-Americans in urban areas? Rural voters who flipped from Barack Obama to Trump? Newly emerging but unreliable young people?

But the one pivotal group showing the most evident signs of splitting from the president are white working-class women, according to a review of polling data, focus groups and interviews with more than a dozen party strategists and voters like Heather.

It's these voters - packed in eastern Iowa, central Minnesota, southwestern Wisconsin, northern Ohio and throughout Michigan - who will wield outsize influence over Trump's 2020 fate. Republican pollster Patrick Ruffini calls these women the essential voter as it relates to the Electoral College.

"If I could talk to one voter in the country, it would be a non-college-educated white woman," he said. "It's definitely the kind of voter Trump can't lose."

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Trump carried non-college-educated white women by 27 points in 2016. They've been slipping away ever since.

In the midterm elections, they were the hidden fuel behind the Democratic comeback. Exit polls showed white women both with and without a college degree shifted 13 points in the Democratic direction from 2016-2018. But white women without a college degree made up a greater percentage of the midterm electorate.

Democrats don't need to carry these voters in 2020, but they must chip away at the president's margins with them in battleground states.

As the impeachment process accelerates, it's white women without a college degree who are gradually warming to the idea that the president's time is up. Forty percent now favor impeachment, up 11 points from mid-September, according to an independent study in late October.

White working-class women make up nearly one-fifth of the presidential electorate, but are far from ideologues of any stripe. Broadly, they care more about their personal incomes than sweeping big-ticket proposals like the Green New Deal. Polls show they respond warmly to raising taxes on the wealthy and corporations and yet are still more likely than other groups to approve of Trump's economy.

It was Trump's positioning as a populist that helped bring them into the Republican fold. Now, more say their wages aren't keeping up with the cost of living, leaving them open to other options.

White working-class men have largely stuck with the president through all of his travails - from Charlottesville, Va., through the entirety of special counsel Robert Mueller's investigation and the current embroilment of an alleged quid pro quo with Ukraine.

But their female counterparts increasingly view Trump as divisive, impulsive and hostile.

"He's definitely lost people because of his rhetoric. There's no cause for it," said Corrie Torres, a teacher in Dubuque, Iowa, and Democratic voter. "I don't think people here will continue to buy into someone who is unkind."

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Driving northbound on Route 63 into Howard County, Iowa, there's a lone placard encouraging adoption on the right and a single pole with an American flag whipping in the wind on the left amid miles of corn fields and farmland. Howard has suffered from a population drain that's precipitated a broad-based consolidation of the schooling system and left local factories struggling to find qualified workers. Politically, it's also felt neglected.

But this cycle looks different. This swath of eastern Iowa is home to 31 swing counties that supported Obama twice before switching their allegiance to Trump - the most of any state in the country.

The largely industrial region - much like its neighbors in Wisconsin and Minnesota - is home to a bastion of white working-class voters who have shown a penchant for choosing individual candidates over long-held party labels.

In Dubuque, the largest county in the area, Trump managed a 16-point shift to win it by just over 1 point. But in no place in the country was there a more dramatic swing than in Howard County, a sparsely populated area along the Iowa-Minnesota border. Four years after Obama's 21-point win, Trump carried it by 20 points in 2016, amounting to the biggest swing in the country.

April Cheeseman is a resident of Lime Springs (population: 477) who used her high school degree to land a job at an electronics manufacturing company in neighboring Minnesota. Trump was never an option for her in 2016, but Clinton repelled her as well.

So she cast her ballot for third-party candidate Gary Johnson, four years after supporting Obama.

This time, she's leaning towards caucusing for Pete Buttigieg next February because of his military service and student debt plan, but will likely support whoever Democrats nominate.

It's a dynamic heard over and over in these counties: 2016 votes were widely driven by a loathing of Clinton, even if they thought little of Trump.

"What I heard on the doors was: 'Hillary? Never Hillary. And even though Trump says stupid s---, he's not Hillary,'" said Laura Hubka, a sonographer and chair of the Howard County Democratic Party, who has endorsed Buttigieg. "I feel the same thing about (Joe) Biden here - people are like, 'nope.'"

Hubka has had her own fissures with the Democratic Party in Iowa, leaving it temporarily after 2016 due to what she saw as unresolvable infighting between the Clinton and Bernie Sanders factions. Yet she's becoming increasingly heartened about a political recovery, in part due to refocused attention on far-flung areas by 2020 candidates.

A June event in Cresco featuring Buttigieg, Amy Klobuchar and now former presidential candidate John Hickenlooper was seen as a breakthrough in the secluded region.

"The candidates have been doing a better job of coming to rural areas, which they never have before," Hubka said, naming Buttigieg and Elizabeth Warren's campaigns as the two doing the most personal interaction in Howard.

In Hubka's eyes, a victory in 2020 would be shaving off at least 10 points of Trump's 2016 margin in Howard County, an outcome that could endanger the president's ability to again carry Iowa's six electoral votes if it was replicated in surrounding counties.

Trump carried Iowa by 9 points in 2016, but there are already signs that it'll be closer this time. A recent New York Times/Siena College poll showed Trump's lead in the state falling to just a single point against Biden and three points over Sanders. But against Warren, Trump's advantage is 7 points.

"Iowans do want a change but they don't want a drastic change. They want something better but ... they don't want a complete liberal takeover," said Sarah Orwig, a Sioux City firefighter who has endorsed Biden. "(Warren) might just be a little too much, to tell you the truth."

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As the polling numbers demonstrate, the profile of the eventual Democratic nominee will matter to these voters, as will the issues he or she chooses to prioritize.

Democratic Gov. Tom Wolf of Pennsylvania, who has said he will refrain from endorsing in the primary, expressed concern that the rigid ideological tests being put forward by candidates is not how most voters come to their opinions on policy.

"I don't think Pennsylvanians go for litmus tests. ... I think Pennsylvanians respond, and I think most Americans respond, to pragmatic things," he said. "There's nothing that is a litmus test, that you're either here or you're there."

While stopping short of criticizing any particular candidate, he added: "I've got to admit, I'm not real clear on exactly what each of the candidates means when they say Medicare for All."

Steve Grubbs, a longtime Republican consultant in Davenport, Iowa, is even more explicit about the type of Democrat who would present the greatest challenge to Trump in the Midwest.

"Buttigieg would be strong, (Amy) Klobuchar would be strong. Biden would certainly be stronger than Warren or Sanders," he said. "In Iowa, a northeast liberal is unlikely to sell well here. There is always some geographic bias."

It's this perspective that has fortified Biden's position as the national front-runner for the nomination, even as Warren and Sanders have conjured up more grassroots energy and the former vice president has lost ground in early nominating states.

Biden has sought to make white working-class voters a central part of his coalition. The "dignity of work" is a regular staple of his stump speech. The former vice president recently chose his hometown of Scranton, Pa., to hammer Trump for rewarding "wealth instead of work" in a county where just 27% of residents hold college degrees.

But he's had stiff competition in the Democratic primary for these voters. The latest Quinnipiac University poll showed Warren overtaking Biden with white working-class voters, with Sanders not far behind. Yet in a general election in Iowa, Biden and Sanders carry these voters, whereas Warren loses them to Trump.

For her part, Warren is making the opposite bet of Biden: That winning back these Trump voters requires a bold message underpinned by broad structural economic change. However, Warren still performs weaker against Trump than Biden in hypothetical general election polls in critical Midwestern states.

"She's made herself more plausible to working-class voters. I don't think it's that they love her now," said Ruy Teixeira, a senior fellow at the liberal Center for American Progress, who specializes in political demographics.

But Teixeira warned that if Warren did not walk back some of her most liberal positions as the Democratic nominee, it would prove to be a "significant vulnerability with white working-class women voters" who feel burned. They wanted to believe Trump's promises but now think "they've been sold a bill of goods," he said, making them more skeptical of grand promises.

"Either party, I think, is just pathetic," said Jackie Tallman, a Cresco, Iowa, resident who voted for Clinton in 2016 but is now leaning toward Trump as she's watched the Democrats endorse a litany of expensive proposals. "I never thought I would be ... but where's all the money going to come from?"

"Making all these promises and doing all this stuff for everybody," she said. "It's so unrealistic. It's a fantasyland."

And then there are Democrats like Cory Booker, who are sounding the alarm that an overreliance on white working-class voters in the Midwest risks ignoring black voters in places Milwaukee, Detroit and Philadelphia who didn't show up at the level Democrats needed in 2016.

"That's how we won as Democrats in 2008, 2012 and most recently in 2018," the presidential candidate said recently during a speech in Washington.

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What's clear is that the Trump campaign has no choice but to keep white working-class women in the fold.

So it recently established an eight-member "Workers for Trump" advisory board that will lead an effort to train blue-collar supporters to phone bank and canvass neighborhoods. The double-pronged strategy is to highlight the administration's economic gains while tearing into the more polarizing parts of the Democratic agenda, including the elimination of private insurance through Medicare for All and of fossil fuels by implementing the Green New Deal.

A Trump campaign aide said that unions will be a critical target for this messaging and that educating them on the ramifications of the Democratic agenda will draw them back into the GOP camp.

Angela Tinsley, a single parent, truck driver and Teamsters member who is leading the Trump campaign's effort in Nevada, said the biggest obstacle to winning converts is overcoming the media environment.

"Unfortunately many of them have been misinformed as to President Trump's current record," Tinsley said. "When you say we have the lowest unemployment ... they are surprised by this, like they've never heard of it."

Tinsley acknowledged that "while not everybody agrees with his language ... the proof is in the pudding."

"Since Trump, I'm now receiving a tax refund every year," she said. "As a single parent, that's like a Christmas gift."

But back in Dubuque, Heather counts herself as someone who is already gone. She now describes herself as a "closeted Democrat" still surveying her options. Recently, she showed up at the city's historic brewing company along the Mississippi River to catch a Buttigieg campaign event.

As she's done more research on her own, she's determined she opposes tax cuts for the rich and favors a government safety net for the neediest, but finds Warren and Sanders too extreme.

"My family would have an absolute fit if they knew I were here right now," she said standing at the periphery of the Buttigieg event as the sun set. "Republican has been my identity for a long time, so it's kind of weird to be on the other side."

Buttigieg showed up 50 minutes late, leaving some onlookers antsy, due to the Monday Night Football game, which was approaching kickoff.

Still, she departed toward the parking lot with Buttigieg's sticker affixed to her blouse, undecided on exactly which Democrat to caucus for, but settled on the type of candidate she wanted.

"He seems like a nice, down-to-earth family guy. I like that," she said of Buttigieg. "He's the antithesis of Donald Trump."

Visit the McClatchy Washington Bureau at www.mcclatchydc.com