“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.’”