The AFL-CIO, which includes 60 unions representing more than 12.5 million workers, endorsed Biden and Vice President Kamala Harris on Friday, the earliest it has ever endorsed in a presidential election, and is hosting Saturday's event.

Biden's frequent appearances at union events, including at a labor conference in Washington right after announcing his re-election campaign, shows how important he thinks the labor movement is to a second term.

Hailed as the most pro-union president in history by labor leaders, he has supported collective bargaining at companies, reversed Trump-era rules that weakened worker protections, pushed to reverse a decades-long decline in union membership, and made it easier for union labor to build bridges and ports around the country.

White working-class union voters were divided on Biden in 2020, but some union leaders say he now has their support.

Some building trades unions, whose members traditionally vote Republican, did not endorse any candidate in 2020 after local leaders couldn't agree over backing Biden or Donald Trump.

Ryan Boyer, head of the influential Philadelphia Building and Construction Trades Council, which represents 50 unions including boilermakers and steelworkers, said Biden's track record as president has erased any doubt among his members about who to support in 2024.

"It's a different electorate this time around," he said. "There is not one labor leader worth their salt in Philadelphia that hasn't recognized just how much President Biden has supported men and women in labor. It's much different."

Union voters helped Biden win critical swing states, including Pennsylvania, Wisconsin and Michigan in 2020, and labor is expected to play an important role in the Democratic Party's grassroots operations in the run up to 2024.

Biden won 57% of union households nationwide in 2020 compared with 40% for Trump, according to Edison Research.

His relationship with labor as president has not always been smooth.

In December, some unions criticized Biden for signing legislation preventing a nationwide rail strike. Separately, the United Auto Workers said in May that it wasn't immediately endorsing Biden because of his push to transition the U.S. into a nation reliant on electric vehicles.

Seth Harris, a law professor at Northeastern University, who until recently served as Biden's top labor policy adviser at the White House, said the president's infrastructure, chips and climate bills, which helped create millions of jobs that do not require a college degree, will help him win over holdouts.

"The building trades... are one of the largest groups representing men who have pursued their careers without a college degree," he said. "A substantial part of the president's economic strategy has been focused on men and women who are not going to get a college degree."

Harris said he expects to see Biden making his case in the months ahead in front of union audiences in Michigan, Wisconsin, Georgia, Arizona and other states.