Big Labor warned Democratic presidential contenders Wednesday against taking union support for granted, adding that Dems would need to be more honest about the party’s record on workers’ rights, reports said.

The president of the AFL-CIO labor union, which represents 12.5 million union workers, addressed a closed-door meeting with representatives from each campaign in attendance before the second round of debates Wednesday in Detroit.

AFL-CIO President Richard Trumka argued that “both parties” needed to take responsibility for U.S. labor laws that benefit corporations to the detriment of the middle class.

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“More often than not, the Republican Party is bad for workers. This president is bad for workers. But let’s be honest about the Democratic Party’s record,” Trumka said.

“We are caught in a web of century-old labor laws that prioritize unchecked corporate greed over all else,” Trumka said, according to the Huffington Post. “We can blame this White House all we want. But this isn’t new.”

“We are caught in a web of century-old labor laws that prioritize unchecked corporate greed over all else. We can blame this White House all we want. But this isn’t new.” — AFL-CIO President Richard Trumka

It wasn't the first time the AFL-CIO has criticized Democrats this year.

In March, the union implored Rep. Alexandria Ocasio-Cortez, D-N.Y., and other lawmakers to not go through with the Green New Deal, claiming the plan to combat the effects of climate change would cause “immediate harm” to millions of union employees and threaten their livelihoods.

As the Democratic Party shifts toward a more progressive identity, Trumka reminded 2020 candidates that unions would no longer support candidates simply because of their party affiliation. Unions historically played influential roles in getting Democrats elected through get-out-the-vote efforts, canvassing and other campaigning methods.

“It’s time to do better,” Trumka said. “I believe you can. I believe you will. And working people are hungry for it. But you can’t offer campaign rhetoric or count on workers’ votes simply because you have a ‘D’ next to your name.”

"You can’t ... count on workers’ votes simply because you have a ‘D’ next to your name.” — AFL-CIO President Richard Trumka

“You need to prove that this party is the one and only party for working people,” he said. “And recognize that unions and collective bargaining are the single best way to make this economy work for everyone.” “Convince our members, and you’ll have the country’s largest and most effective movement for working people on your side,” Trumka added.

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Trumka argued that Democrat-backed trade deals, including NAFTA (North American Free Trade Agreement) and TPP (Trans-Pacific Partnership), hurt the middle class, the Huffington Post reported.

The TPP, an Obama-era global trade deal that was never submitted to the Senate for approval, was signed by 11 nations: Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam. President Trump pulled support from the deal soon after his inauguration in January 2017, arguing it gave up American power. Supporters argued the agreement would benefit the American middle class by making it easier for small business owners to sell American-made goods abroad, Politico reported.

The AFL-CIO has yet to endorse a candidate ahead of the 2020 election.

Fox News' Louis Casiano contributed to this report.