“Unions are here to stay!” Massachusetts Sen. Elizabeth Warren exclaimed Wednesday to an enthusiastic crowd as leaders and members of construction unions packed a Washington Hilton ballroom to hear from nine Democrats running for president or thinking about it.

The audience at the North America’s Building Trades Unions conference heard promises to boost spending on infrastructure, expand apprenticeships, and redirect money that went for tax cuts in 2017 toward the middle class.

Unions can provide campaigns with both contributions and personnel for canvassing and phone banks, and their leadership tends to overwhelmingly back Democrats. But President Donald Trump’s pledges in the 2016 election to focus on worker issues, including renegotiating trade deals, helped him attract some rank-and-file labor support.

Watch: 2020 Democrats at labor event can’t stop talking jobs

Democrats gearing up for the 2020 presidential primaries tried to win those voters back with promises to end right-to-work laws, boost spending spending on infrastructure and make sure the workers building it were paid prevailing, usually union, wages.

Warren energized the crowd by railing against government corruption and accusing Trump of mistreating working-class Americans. She promoted her proposed 2 percent tax on income that exceeds $50 million, which she would invest in union jobs, building roads, bridges and homes.

“Let’s face it — Washington is corrupt, and this economy is working for the rich and the powerful and not for anyone else, but we have the power to change that. We have the power to make big, structural change,” she said.

Nearly every speaker expressed concern that working-class Americans were being overshadowed by big corporate interests who have immense influence on the government.

Former Colorado Gov. John Hickenlooper focused on income inequality.

“We’re a hourglass economy with a large number stuck at the bottom, a hallowed out middle and a smaller, growing top of sky-high incomes of wealth,” he said.

The crowd roared after California Sen. Kamala Harris mentioned that some Americans need two jobs to survive.

“In America, nobody should have to work more than one job to get through the end of the month!” she said.

Several candidates focused on the nation’s partisan divide.

“I want leaders that are about uniting Americans — all Americans, pulling us together to do what must be done. … Fighting fire with fire is not a good strategy,” New Jersey Sen. Cory Booker said.

Booker proposed a $2 trillion infrastructure plan that would be paid for by undoing the 2017 tax overhaul Trump and Republican majorities in both chambers enacted.

Minnesota Sen. Amy Klobuchar assured workers that she had their backs after proposing a trillion-dollar infrastructure package, also funded by reversing tax breaks for upper-income brackets.

“We have to shore up the cracks of Americans infrastructure,” she said.

At a focus group meeting after the speeches, some union leaders were concerned candidates did not provide specific policy plans. There was also skepticism about the federal government’s ability to pass a bipartisan infrastructure package. But some were swayed.

“Elizabeth Warren really impressed me. … I think she understands the fundamentals of what out members are concerned about right now,” said Vince Sugrue, a representative from Sheet Metal Workers 104 in California.

Reps. Tim Ryan of Ohio and Eric Swalwell of California, who entered the presidential race earlier this month, also spoke, as did two potential candidates, former Virginia Gov. Terry McAuliffe and Colorado Sen. Michael Bennet.