The prospect of a Donald Trump nomination has labor leaders scrambling to hold the line as the Republican frontrunner’s appeal to disaffected working-class voters threatens to upset the traditional political calculus.

The majority of America’s almost 15 million unionized workers can be usually be relied upon to back the Democratic candidate in a presidential year, but leaders are concerned by Trump’s populist message on trade and jobs – and his insistence that union workers are just one of many groups on a long list of those he claims “love” him.

“We can’t be fooled,” Richard Trumka, president of AFL-CIO, America’s largest federation of labor unions, implored his 12.5 million members at the organization’s annual convention in Philadelphia this month. “Trump isn’t interested in solving the problems he yells and swears about. He delivers punch lines, but there’s nothing funny about them.”

As Hillary Clinton looks to push away the threat from Bernie Sanders with further wins in a slate of Democratic primaries across the north-east on Tuesday, organized labor is planning a multi-pronged assault on Trump in an effort to undercut his appeal and derail his presidential bid to the White House.

“Trump has some appeal at this point, there’s no question about that,” said Steve Rosenthal, former political director for AFL-CIO. “But when you cut through it and begin to focus on his record – from his talk about trade agreements, to manufacturing abroad to offshoring jobs – Donald Trump is not going to appeal to union members.”

In the coming months, the AFL-CIO, which has not endorsed a candidate in the primary but has encouraged members to support the Democratic nominee, will launch digital attack ads against Trump and will ramp up its door-knocking campaign. The Service Employees International Union, which has endorsed Clinton, has organized phone banks for her nationwide, including most recently in Pennsylvania in the hope of getting out the vote on behalf of Democrats.

Yet Trump is resonating with voters who are struggling to make ends meet and who are seeing their friends’ jobs shipped abroad, says John Cakmakci, president of United Food and Commercial Workers Union Local 951 in Michigan. And some of those voters are union members. Trump’s populist positions on trade and his rejection of Washington politics have earned him votes across the Rust Belt, where several battleground states are key to winning the election in November.

UAW had come out in support of Obama in both 2008 and 2012. Photograph: Chip Somodevilla/Getty Images

“[Trump] portrays himself as someone who is going to get it done: ‘I am going to get these jobs back.’ But the fact of the matter is: how are you going to get them back? But people don’t want to know that. They just like that five-second simple solution to the problem,” he said. He added that Trump’s promises appeal to both Republicans and Democrats.

While unions still tend to lean Democratic, their rank-and-file members are increasingly split between the parties. In 2008, about 39% of households with union members cast their vote for Republican John McCain and in 2012 40% voted for Mitt Romney.

Working America, the political organizing arm of AFL-CIO, wanted to find out whether Trump’s “rightwing” message appealed to workers outside Cleveland and Pittsburgh. After interviewing about 1,689 working-class Americans living in households earning less than $75,000, they found out that Trump was in fact the favored candidate. Of the 800 voters who had decided on a candidate at the time of the interview, about 300 favored Trump. Combined, the two Democratic candidates appealed to fewer workers – 174 chose Clinton and 95 chose Sanders.

“While most of Trump’s support comes from the staunch Republican base, one in four Democrats who chose a candidate showed a preference for Trump,” said Working America’s report.

The majority of respondents said they supported the candidate because of his pugnacious personality rather than for, say, his position on trade.

On Tuesday, Jared Szczesny, a card-carrying member of the United Automobile, Aerospace and Agricultural Implement Workers of America Union (UAW), will cast his vote for Trump in Pennsylvania’s primary. In 2008 and 2012, UAW endorsed President Obama. This time UAW has yet to endorse a candidate, but is likely to back a Democrat.

Szczesny, 31, has never attended a Trump rally. He works seven days a week and has not been able to find the time. However, back in October, he picked up Trump’s book The Art of the Deal. When he finished it, he knew that Trump had his vote.

Democratic US presidential candidate Bernie Sanders speaks during an election rally in Erie, Pennsylvania on 19 April. Photograph: Lucas Jackson/Reuters

About 127 miles north of Pittsburgh and 100 miles east of Cleveland is Erie, Pennsylvania, where Szczesny lives. He works for Accuride Wheel End Solutions, where he spends his afternoons at the plant, operating a machine that cuts wheels from steel and aluminum.

Erie is one of the cities left behind by the US economic recovery. In 2014, the median household income in Erie city was $33,007, and more than 27% of its residents were living in poverty.

“I work a half of a block from GE, where layoffs come in the thousands every couple of years or so,” said Szczesny. GE Transportation is part of General Electric; in November the GE locomotive manufacturing plant in Erie laid off 1,500 of its 4,500 workers. “There are no jobs around here. People scrounge for work, or work multiple jobs. I get tired of seeing my family and friends, and loved ones struggling over financial issues so often.”

It is Trump’s business acumen that appeals to Szczesny and a handful of his coworkers, who he says will also be voting for Trump.

“He is not a typical politician. He has been in the working world, even as a young boy building houses with his father. He’s not a career politician. He cares about jobs,” he said.

Earlier this month, Trump stood before nearly 9,000 supporters in Pittsburgh and promised to revive the steel and coal industries in the area if he became president.

“There are few places that have been more devastated by our trade policies than Pittsburgh. Don’t worry, we’re bring it all back,” said Trump.

Szczesny hopes so. It’s why when he votes in Pennsylvania’s primary on Tuesday, it will be for Trump.

“I am very conservative and do not agree with everything Mr Trump says or does, but I recognize him as the best candidate, and a great one at that,” he said.