Though it showed a virtual tie, the NABTU survey represented a seven-point drop for Trump from March, the first change for the president in two years, said Sean McGarvey, the union president. He attributed the change to Trump’s management of Covid-19, calling it “the greatest mistake of his presidency.”

“It’s going to be close among my members between Biden and President Trump, but there’s been dramatic change in the last six months,” McGarvey said, adding that if Trump had been more aggressive toward the coronavirus, “He’d be bulletproof. We wouldn’t even be talking about Joe Biden now.”

However, even as they praise the Democratic presidential nominee as a less-flawed candidate than Clinton, other union leaders said they fear there’s nothing they can say to the Trump supporters among their ranks to sway their opinion between now and November.

They said parts of Biden’s record, such as his past support for the North American Free Trade Agreement and the Trans-Pacific Partnership, hurt him, and that some members still look to the pre-pandemic economy under Trump as a high point.

“It doesn’t seem like there’s anybody changing their minds,” said Don Furko, president of the United Steelworkers Local 1557 in Clairton, Pennsylvania, who said the majority of his membership is backing Trump.

In northern Minnesota, local USW officials are working to educate members on steps Trump has taken to attack organized labor and encouraging them to vote for their jobs rather than on social issues like immigration, said John Arbogast, staff representative for District 11. But, he added, “you’re not going to change a Trumpster’s opinion.”

TJ Ducklo, Biden’s national press secretary, said the former vice president “sees this election as Park Avenue vs. Scranton — a choice between someone who has always stood up for working people and believes this country was built by unions, and Donald Trump, who punishes the middle class with tax cuts that only benefit him and his rich friends.”

Samantha Zager, deputy national press secretary for the Trump campaign, said, “Throughout his 50 years in politics, Biden has consistently put special interests ahead of American workers — including in his promise to shut the economy back down if he were elected — and that’s why President Trump is seeing strong support from union members.”

At a national level, most union officials have largely lined up behind Biden, appearing with him at events and acting as campaign surrogates. But they said their efforts to spread that enthusiasm have been hindered by the pandemic, which has severely limited organized labor’s in-person ground game.

With the exception of Unite Here, most unions have avoided door-knocking in an effort to prevent spreading the virus. Labor officials said some members don’t share their cell phone numbers, which is a challenge when their efforts to boost Biden have shifted to phone-banking, texting and other virtual work. So-called labor walks — organized labor’s term of art for canvassing — are largely no more.

Pat Eiding, president of the Philadelphia AFL-CIO, said trades leaders need to talk with their members more often about Biden to be effective. A refinery closed recently in the city and Trump supporters have misleadingly made some union workers believe he'll bring back the facilities, he said.

The evidence that member outreach works, he said, is the 2008 presidential election, when they spoke frequently with the rank-and-file at job sites as early as 5 a.m. and overcame prejudice in their unions against an African-American Democratic nominee.

“We said ... the man’s Black. But here's the issues: He’ll protect you and your jobs,’” he said. “And when we were able to go directly to members, instead of just being on a bully pulpit, we were able to get them to vote for Obama. And that was not an easy thing to do.”

Furko said the USW’s political team is knocking on doors, but it has not come to distribute materials and share information about Biden at the plant and union hall as they normally would due to the virus — they simply sent fliers instead.

“They know that they’re there if people want to come,” he said. “But I still have at least 2,000 pieces of paper on my table.”

In a sign of the tension among construction workers about Trump, at least one building trades union has not yet endorsed a candidate in the presidential race: the International Union of Operating Engineers.

“Everybody seems to be dancing on this thing. But if you go by my house, you’ll see on the lawn a Biden sign,” said Robert Heenan, second vice president of IUOE. Among rank-and-file members in Pennsylvania, where he is business manager of Local 542, he added, “I think a lot of them are going for Trump.”

The United Mine Workers of America have also held back an endorsement. Members turned against Clinton in 2016 when she said she would put coal miners out of business, and many are staying away from Biden because they fear he is also anti-coal, said Chuck Knisell, an international vice president of UMWA’s District 2.

“The biggest argument that I have from our membership is that this isn’t a blue-collar, working-class Democratic Party that my dad or mom was in,” Knisell said. “It’s morphed into something different.”

But within more liberal unions, especially those with large numbers of Black and Latino members, leaders report major excitement for Biden — which could potentially offset GOP gains in the building trades as well as other moderate- and conservative-leaning unions. Public-sector unions in major cities likewise tend to be a font of Democratic support.

Mary Kay Henry, president of the Service Employees International Union, said 70 percent of her rank-and-file members backed Clinton. This year, she said, at least 80 percent are supporting Biden.

“And his pick of Kamala Harris has probably moved the needle — we’re just going back into the field with a poll — to 90 percent,” she said. “Our members are on fire for the Biden-Harris ticket.”

Union officials said there is also some evidence that Biden may be making inroads among another camp of rank-and-file members: those who disliked both candidates in 2016 and chose to vote third party.

Labor leaders have zeroed in on those members as the most persuadable this year. Some estimate that third-party voters in the union ranks were as big a problem for Clinton as the voters who flipped to Trump, so winning that cohort alone could give Biden a significant boost.

“Going into this election, those folks who sort of felt like they couldn’t vote for Trump in 2016, but really weren’t enthusiastic about Clinton, really understand what’s at stake now,” said Michael Podhorzer, political director with the AFL-CIO. “I think that’s something of a bump that can be counted on.”

Ryan Bennett, who’s in charge of roughly 800 active and 200 retired plumbers and pipefitters as part of UA Local 174 in Coopersville, Mich., said he’s starting to hear from members who view the past three-plus years of the Trump administration as a series of broken promises for organized labor.

“They’ve all seen that none of that has come to fruition,” Bennett said, referring to Trump’s pledges to bring back manufacturing jobs and overhaul the country’s infrastructure. “A lot of those members that were in the middle are going to end up on Joe Biden’s side.”