At a Fox News town hall in Bethlehem, Pennsylvania, on Monday night, Sanders mused to the moderators that President Donald Trump, who "watches your network a bit," might be tuned in for the festivities.

The morning after and again on Tuesday night, Trump confirmed Sanders' well-founded suspicion with a series of snappish tweets, alleging the network had "stuffed" the room, which applauded many of the Vermont independent's answers, "with Bernie supporters," after initially -- about 10 hours earlier -- calling it "so weird to watch Crazy Bernie on Fox News."

Indeed, the sight of any Democrat -- much less a democratic socialist running for the party's presidential nomination -- in a venue so closely aligned with the Trump administration, is an unusual one. Sanders' decision to tread those uneven boards upset more than a few liberals commentators, who worried that his presence alone might gift the network some undeserved credibility. For Trump, though, the notes of aggravation in his tweets pointed to a broader new conflict: Here, for the first time, one of the candidates vying to deny him a second term had stood up on his stage, in front of an audience Trump views as his own, and delivered an unflinching case against him.

Sanders, who capped off his four-day, five-state trek through the Upper Midwest with the Fox News event, believes his path to the White House runs directly through Trump Country. And he will not be deterred by a boisterous president or Democrats concerned that his focus on the region risks playing into what they view as a distorted reading of the 2016 election -- one that suggests "economic anxiety" drove voters to Trump and not the President's open appeals to race-based resentments. Hillary Clinton, after all, won nationally with union households (51% to Trump's 42%) and voters making under $50,000 a year, according to exit polls.

"I can understand why people voted for Trump based on what he said (in 2016)," Sanders told supporters at a windswept rally in Madison, Wisconsin, last Friday. "And the reason for that, is that in Wisconsin and all over this country, there are a lot of people who are hurting. There is a lot of pain out there."

It was an argument Sanders repeated throughout the trip, one rooted in a belief that at least the outer rungs of Trump's base, a cohort viewed by some Democrats as too far gone to be won back, are there for the taking -- if only Democrats confront them with the reality of Trump's record in office.

"Trump lied when he said that he would listen to their pain," Sanders said. "So our job in this campaign is to reach out to working families who are hurting and to work with them to finally create a government and an economy that works for all of us and not just the 1%. That is what the American people want -- they're tired of being lied to."

For many on the left, particularly those in the organizing trenches, Sanders' view is a recognizable one. And it fits with their own analysis of the 2016 election, which holds that a decisive, if relatively modest, number of frustrated swing voters either opted for Trump's right-wing populism or stayed home because of the absence of the left-wing alternative.

"If you're going to a state that was won by Trump, you have to reckon with what drove voters to do that," a Sanders aide said ahead of his speech in Warren, Michigan. "If you're in Green Bay, Wisconsin, if you're in Kalamazoo, Michigan, if you're in Wilkes-Barre, Pennsylvania, you know what NAFTA did to your community, you know what these tax bills have done to your community."

A few minutes later in his remarks, Sanders came out for the first time against Trump's renegotiated version of the trade deal, the administration's United States-Mexico-Canada Agreement.

"I challenge Donald Trump, for once in your life, keep your campaign promises," Sanders said. "Go back to the drawing board on NAFTA, do not send this treaty to Congress unless it includes strong and swift enforcement mechanisms to raise the wages of workers and to prevent corporations from outsourcing American jobs to Mexico."

But the underlying logic, promoted by both Sanders' campaign and the candidate himself, that says some Trump voters can be peeled off and brought back across the aisle, remains largely unproven in more recent elections.

Democrats won back the House during the 2018 midterms -- and claimed or retained governor's mansions in Michigan, Wisconsin and Pennsylvania -- by marshaling a coalition of super-charged moderate liberals. But Trump himself was not on the ballot and most Sanders-backed progressive candidates either lost their primaries or fell victim in November to the kind of attacks likely to be leveled at any eventual Democratic presidential nominee.

Down in Florida, Democratic gubernatorial candidate Andrew Gillum, who is African American, led in most polls leading up to Election Day last year, typically outpacing now-former Democratic Sen. Bill Nelson, who is white. But they both lost to their Republican opponents, who got eleventh hour boosts from Trump, and Nelson ultimately won more votes than Gillum, the former Tallahassee mayor Sanders endorsed before the primary.

Back up north, Democratic candidates across the primary field are beginning to sharpen their efforts to rebuild the so-called "blue wall" that Trump smashed through in 2016. Pete Buttigieg, the young mayor of South Bend, Indiana, offered a preview of his own argument at his campaign's formal kick-off this weekend.

"There is a myth being sold to industrial and rural communities," Buttigieg said, leaving the best known salesman unnamed, "the myth that we can stop the clock and turn it back."

"It comes from people who think the only way to reach communities like ours is through resentment and nostalgia," he continued, "selling an impossible promise of returning to a bygone era that was never as great as advertised to begin with."

But Sanders has been the most direct. An April 8 memo from his campaign managers stated it plainly, arguing that "the 2020 Electoral College map shows the most direct path to a Democratic victory is flipping back Wisconsin, Michigan and Pennsylvania" -- three states Clinton lost in 2016 by fewer than 78,000 votes combined.

Democratic pollster Celinda Lake sees opportunity and risk in Sanders' tactics.

"The irony," Lake said, "is that Sanders was the future in 2016, but he has to be careful not to be the past in 2020."

But she praised his approach, in saying he "understands" why people chose Trump, as a practical decision.

"It's really good that Bernie Sanders said that because I think every candidate is going to have to end up saying that," Lake told CNN. For Democrats, she added, "I think this is the beginning of saying, 'I understand what you were voting for, you didn't get it and I'm proposing something that will get you what you need.' But to just tell voters they were wrong isn't a very successful way to persuade them."

Sanders' combative performance at the town hall in Bethlehem, where he was confronted with a mix of standard-issue campaign questions and Trump talking points, will hearten his loyalists. It also underscored the theory, set out by campaign manager Faiz Shakir in Pittsburgh a day earlier, that Sanders is the Democratic primary candidate best equipped to withstand the coming Republican onslaught.

"Donald Trump is going to say every candidate is a quote-unquote 'socialist.' Whether you're Kamala Harris or Pete Buttigieg or anybody else, he's going to say that about you," Shakir said, asserting that Sanders -- who is happy enough to talk about his democratic socialism when asked -- won't "flinch" when faced with the line.

"When that (response to GOP attacks) comes out of the candidate's mouth, who are voters going to trust and believe?," Shakir asked, before answering his own question: "On that score, I believe that when Bernie Sanders says those things, people are going to say, 'Yeah, I believe that he's going to do those things.'"