The unions have highlighted business practices at Mr. Trump’s hotels and resorts that they deem anti-union, including efforts against unionizing Trump International Hotel in Las Vegas and the hiring of foreign workers at Mar-a-Lago Club in Palm Beach, Florida.
But the real estate mogul’s popularity with the union’s white, blue-collar base has persisted.
The national Ironworkers Union endorsed likely Democratic nominee Hillary Clinton.
A local union leader in Dayton, Ohio, gave a similar assessment of divided membership, except he described a three-way split between Mr. Trump, Mrs. Clinton and her far-left rival, Sen. Bernard Sanders.
He said the level of support for Mr. Trump among members in his construction trades union was “more than you think.”
“There’s a lot of people fed up with Washington politics altogether,” said the union official, who asked to remain anonymous when talking about internal political divisions. “It’s an extraordinary election. I just don’t know how it is going to go.”
His national organization also endorsed Mrs. Clinton.
The only trade unions that have endorsed Mr. Trump are the National Border Patrol Council and the New England Police Benevolent Association. And yet rank-and-file members from throughout the union movement flock to Mr. Trump, even from organizations such as Service Employees International Union, whose activists have been at the forefront of street protests denouncing the billionaire businessman.
The attraction of Mr. Trump’s populist, tough-on-trade message for union members who traditionally vote Democrat isn’t the only problem vexing union leaders in the unpredictable 2016 race. They also are struggling to mend the rift between national organizations that overwhelmingly back Mrs. Clinton and local unions that have sided with Mr. Sanders, who, like Mr. Trump, has tapped into a powerful anti-establishment strain in the electorate.
“Right now those voters are coalescing around Sanders as much as they are coalescing around Clinton. The problem still has to be worked out,” said Kate Bronfenbrenner, director of Labor Education Research at the Cornell University School of Industrial and Labor Relations.
She predicted union voters would ultimately come home to the Democratic Party in November.
“Whatever gets worked out between Clinton and Sanders, they are not going to go Trump,” said Ms. Bronfenbrenner. “But Trump is a force to be reckoned with, and it’s not going to be an easy election because he has stirred up hate and he’s stirred up this racist elements, and he’s stirred up these economic fears in this country and this idea that maybe some guy can come along and make everything better again.”
Mr. Trump has boasted that his popularity with blue-collar workers or “Reagan Democrats” will expand the electoral map for the GOP by putting in play union strongholds such as Pennsylvania and Michigan.
He has struck a chord with working-class voters by promising to negotiate better trade deals, punish companies that move manufacturing operations to Mexico or other cheap-labor destinations and to end President Obama’s anti-coal policies.
Polls also have shown that among white union households, support is evenly split between Mr. Trump and Mrs. Clinton, a sharp break from recent elections. President Obama won 59 percent and 58 percent of the vote of union households in his 2008 and 2012 presidential runs, respectively. Even Ronald Reagan lost union households to Walter Mondale by a narrower 54 percent to 46 percent in his 1984 landslide win, according to the New York-based Roper Center for Public Opinion Research.
That explains why AFL-CIO President Richard L. Trumka implored workers to resist Mr. Trump at the federation’s April convention in Philadelphia.
“We can’t be fooled. Trump isn’t interested in solving the problems he yells and swears about,” he said. “He delivers punch lines, but there’s nothing funny about them.”
Mr. Trumka said the billionaire businessman loves right-to-work laws, routinely mistreats workers at his own companies and cheered Wisconsin Governor Scott Walker’s state pension system reforms.
“And he says our wages are too high. Let me repeat that: He says our wages are too high,” he said. “These facts get too little attention, and that’s why we’ve got to constantly expose Donald Trump for what he is: dangerous, delusional and a demagogue.”
The AFL-CIO is revamping its anti-Trump campaign with a renewed focus on the real estate mogul’s comment during a GOP candidate’s debate in November that American workers’ wagers are “too high.”
Kent Wong, director of the Labor Center at the University of California, Los Angeles, said he expected the national union leadership would escalate the anti-Trump campaign to an unprecedented level of activism.
“There is tremendous opposition to Trump’s message, and a lot of the racist and sexist remarks that Donald Trumphas made on the campaign trail have very much been opposed by union leaders,” he said. “I do think there will be more activism this year if he proceeds as the nominee.”
“In western Pennsylvania I see that happening,” said Larry Nelson, an organizer for the Building and Construction Trade Council in Beaver County, which borders the Ohio River about 35 miles northwest of Pittsburgh. “The people are definitely going toward Hillary. For the rest of the country I’m really not sure.”
Mr. Trump’s own rhetoric was more responsible than union leaders’ call for members to stick with the Democratic candidate.
“The way he speaks — I think it kind of puts people off a little bit,” said Mr. Nelson.
Meanwhile, other Republican candidates are garnering support from unions.
Sen. Rob Portman last week picked up the endorsement of the United Mine Workers of America’s political action committee in his tough reelection run in Ohio.
The union said that it recognized the support Mr. Portman had “given both active and retired coal miners and their families, especially in such difficult times as the coal industry is experiencing today.”
UMW’s decision to pick Mr. Portman over the Democratic challenger, former Ohio Gov. Ted Strickland, whom the union previously endorsed for governor, underscored the damage Mr. Obama’s environmental policies have inflicted on the Democratic Party’s relationship with the union.