President Trump delivers remarks at a conference of North America’s Building Trades Unions in D.C. on April 4. (Olivier Douliery/European Pressphoto Agency)

Seconds before President Trump took the stage at the annual legislative conference of the building trade unions, a lone “boo” rang out at the mention of his name.

After receiving better-than-expected support from union members for his populist, anti-free-trade message, Trump made a hard pitch to the members of an assortment of union groups most likely to be positively disposed to his message.

“Did you ever think you’d see a president who knows how much concrete and rebar you can lay down in a single day?” Trump asked. “Believe me, I know.”

“We’re a nation of builders and it’s about time you had a builder in the White House,” Trump added, prompting applause from the gathered members.

But minutes later, he traveled a well-trod path, recounting his election victory over his Democratic opponent Hillary Clinton.

“The electoral college is very, very tough,” Trump told them, noting that he had won Wisconsin and Michigan, two states he “didn’t even need.” “They say almost impossible for a Republican to win.”

As he began criticizing Democrats, the crowd shifted in discomfort.

“I don't know if you saw, but if you watch the Democrats, the anger, and they're trying to figure out, where did all these people [come from] that voted for Donald Trump,” he added.

“I had the support, of I would say, of almost everybody in this room,” Trump said. “I had tremendous support of the workers.”

“Nope,” a few voices called out. Others followed with “boos” and chuckles.

“Would you like to make a change, folks?” the president asked. “Would you like to make a change?”

The trade unionists have been unusually enthusiastic about Trump mostly because of his promise to rebuild the country’s infrastructure and restart construction on oil and gas pipelines and renegotiate free trade deals. And on Tuesday Trump was eager to remind them that he had approved the controversial Keystone XL pipeline and officially killed the prospects for the Trans-Pacific Partnership trade deal, both lines that got him robust standing applause from the audience.

Trump has courted the support of union leadership, which typically allies with the Democratic Party and which has kept him at arm's length despite his stronger-than-expected showing among union members in the election.

Union leaders have openly said they are willing to work with Trump on policy issues that they support, like infrastructure and trade deals. But they remain wary of his more controversial policies, including a travel ban for people from predominantly Muslim countries and his push to repeal the Affordable Care Act.

Just minutes into his speech, a group of about half-a-dozen conference attendees stood and brandished white pieces of paper with the word “#RESIST” printed on them. They were quickly escorted out of the room.

Trump, hoping to spur more widespread support among union members and their leaders, has had several of them to the White House since his inauguration.

Sean McGarvey, president of North America's Building Trades Unions, has been effusive in his praise of Trump.

Introducing him Tuesday at the group’s legislative convention, McGarvey said Trump is “the very definition of an American success story.”

Beyond his promises to pursue infrastructure investments, Trump has also appealed to workers with his promises to make the economic system more fair for people at the bottom.

Though the president has assembled one of the wealthiest cabinets in history, speaking to union members a few minutes north of the White House, he criticized politicians and Wall Street for reaping all the benefits of the country’s economy.

“Washington and Wall Street have done very, very well for themselves,” Trump said. “Now it’s your turn, and you're going to be also sharing the wealth.”

“I promise you America's labor leaders will always find an open door with Donald Trump,” he pledged.