David Goldman/ASSOCIATED PRESS Republican presidential candidate Donald Trump gives a thumbs-up to supporters during a rally on Feb. 9, 2016, in Manchester, New Hampshire. Union leaders are concerned about Trump's growing appeal for working-class voters.

Over the course of five weeks, Working America, the non-union affiliate of the AFL-CIO labor federation, did extensive canvassing in union-dense, blue-collar areas of Pittsburgh and Cleveland. They called it a "front porch focus group." The idea was simply to listen -- to let likely 2016 voters sound off about their thoughts and concerns headed into the presidential election.

What they discovered, among other things, was a lot of support for Donald Trump, the GOP front-runner. For months, this enthusiastic backing of the obnoxious billionaire had generally baffled the chattering class -- not to mention the GOP and Democratic establishment. But to Working America canvassers, it made plenty of sense.

"We hear the same refrains all the time," said Karen Nussbaum, executive director of Working America, which has high membership in the Rust Belt. "That people are fed up and they're hurting. That their families have not recovered from the recession. That every family is harboring someone still not back at work. That someone is paying rent for their brother-in-law."

"And then a guy comes on the stage," Nussbaum explained, "and says, 'I'm your guy who will blow the whole thing up.'"

Trump's pyromaniac approach to politics has earned him strong support from white, working-class voters and brought him to the cusp of winning the GOP nomination. It is an ascent that has shaken Republicans, who view the businessman as a fraud bound to splinter the party, and it's leading Democrats and their allies to do what they do best: fret and panic.

Trump, the worry goes, is making precisely the right appeals at precisely the right time to fundamentally realign the Rust Belt working class electorate's traditional political allegiances.

"In terms of his message, it is really resonating. Particularly if you are talking [about] union people, he is speaking our language," said Josh Goldstein, deputy national media director for the AFL-CIO. "We can't let that go unattended, because people have been doing that with Trump for a long time, and his numbers have only gone up. ... It is our job to go out and educate people now, so it doesn't cross that threshold and become a threat."

High-ranking labor officials are becoming increasingly outspoken in their warnings about the Republican front-runner. Earlier this week, Terry O'Sullivan, head of the powerful Laborers' International Union of North America, attacked Trump as a "racist, sexist, prejudiced billionaire bully." The members of O'Sullivan's union tend to work in construction, the sort of demographic for which Trump's economic message can resonate.

At an AFL-CIO executive council meeting last month, officials vowed to start digging in more aggressively on the records of the Republican field -- and Trump in particular. The federation has since launched a digital ad campaign, while its president, Richard Trumka, has traveled across the country to deliver speeches in union halls and talk individually to union members. He has called Trump an anti-American "bigot" who's full of "baloney and bluster."

"He starts with a different profile than George Bush or Mitt Romney," Andy Stern, former president of Service Employees International Union, said of Trump. "He is the first Republican in a while that has real appeal. I don't think people looked at Mitt Romney and said, 'He's going to fight for me.'"

What worries Stern, and many officials in the labor movement, is that Trump's appeal to working-class voters is more than just a byproduct of his master showmanship. Trump's denunciations of trade deals, his condemnation of politicians who ushered in outsourcing, and his tough, often-xenophobic rants about immigrants taking domestic jobs all lay out a policy portfolio that, at the most basic level, can be attractive to the economically marginalized.

He is the first Republican in a while that has real appeal. I don't think people looked at Mitt Romney and said, 'He's going to fight for me.' Andy Stern, former SEIU president

These days, Republican presidential candidates generally take a hard line against unions, advocating policies that would further diminish organized labor's role in the U.S. economy. But Trump's angle isn't so clear. He's voiced support for anti-union right-to-work laws while on the campaign trail, but he's also bragged about having good relationships with unions as a businessman.

"He can draw on a well. And I just don't know which well is he going to play in the general," Stern said. "Is he the anti-minimum wage, anti-union, pro-right-to-work [candidate]? Or does he become the I-love-unions [candidate]?"

Larry Cohen, the former longtime president of the Communications Workers of America, said there's a lot for Democrats and unions alike to learn from Trump's rise. First and foremost, they should acknowledge the populism he has tapped into if they don't want Trump to win the White House.

"I think the key will be the Democratic Party has to show that it can be a populist party, not a party of the corporate elite or the establishment," said Cohen, who endorsed Democratic hopeful Bernie Sanders and has actively campaigned for the Vermont independent. "It depends not only on who the [Democratic] candidate is, but what kind of convention we have and what kind of platform we have. Right now, for good reason, working people are skeptical of the authenticity of the Democratic Party."

"If the Democratic Party seems to be a populist party and becomes a populist party, Trump will get crushed," Cohen added. "Unions, regardless of who the nominee is, need to quickly become a vehicle for reform inside and outside the Democratic Party, or they're going to lose significant numbers of their own members this election."

According to Nussbaum, the right message can neutralize Trump on his very own bread-and-butter issues. Take outsourcing: With his own clothing line, Trump has taken advantage of the same cheap overseas labor that he's criticized other U.S. companies for using. And despite his aggressive stance on immigration, Trump has been happy to bring in foreign guest workers on visas to work on his properties in Florida. Those facts resonate with voters, according to Nussbaum.

"It doesn't take too much to be able to have a conversation with most of these folks and say, 'Really, will [Trump] solve all the problems?'" she said. "There's an opening here to move people back to places that focus on issues and not just the showiness."

Right now, for good reason, working people are skeptical of the authenticity of the Democratic Party. Larry Cohen, former Communications Workers of America president

Outside of the union movement, there is broader disagreement among Democrats over how close Trump is to actually becoming a major threat. Ed Rendell, the former governor of Pennsylvania and Democratic National Committee chair, was unalarmed about the prospect of facing the real estate tycoon in a general election. Trump would have appeal to working-class, blue-collar voters, Rendell conceded. But in his home state, those voters weren't as determinative as they were a generation ago.

"Even assuming there are still a block of Reagan Democrats, for everyone we lose because of Trump's candidacy, we will win one independent and one moderate Republican in the suburbs," Rendell predicted. "So I think the trade-off is a significant plus. If it is Trump versus Hillary [Clinton], Hillary will roll up historic margins in the suburbs. Some would be for her. But others will be people voting against Trump."

But not everyone shared Rendell's optimism. Jim Kessler, senior vice president for policy at the centrist Democratic think tank Third Way, argued that Trump was an asymmetrical challenge for the party.

"He’s unlike a candidate like [Sen. Ted] Cruz, in which you can predict with assuredness where he will play or fall flat. Trump is a variable who has exceeded expectations," Kessler said. "Until he stops exceeding expectations, I will worry."

Worrying is what makes Kessler a Democrat, we reminded him. He replied: "It does!"

Editor's note: Donald Trump is a serial liar, rampant xenophobe, racist, misogynist, birther and bully who has repeatedly pledged to ban all Muslims -- 1.6 billion members of an entire religion -- from entering the U.S.