ATLANTA (AP) -- Workers at auto plants in the South should be free to unionize without pressure from employers or anti-union governors, acting U.S. Labor Secretary Julie Su said Thursday, even as some southern states pass laws to inhibit organized labor.

"That choice belongs to the worker, free from intervention, either by the employer or by politicians, free from retaliation and threats," Su told The Associated Press in an interview Thursday in Atlanta. "And what we are seeing is that workers who were thought to be too vulnerable to assert that right are doing it, and they're doing it here in the South."

The United Auto Workers union vowed a broad campaign to organize southern auto assembly plants after winning lucrative new contracts in a confrontation with Detroit's automakers. Last week, 73% of those voting at a Volkswagen AG plant in Chattanooga, Tennessee chose to join the UAW. It was the union's first in a Southern assembly plant owned by a foreign automaker.

Workers at Mercedes factories in Tuscaloosa, Alabama, will vote on UAW representation in May, and the company has also targeted plants in Georgia, Mississippi, South Carolina and Texas.

But political and business leaders in southern states have long fought organized labor. Ahead of the Volkswagen vote, six Southern Republican governors criticized the UAW's organizing drive, arguing that autoworkers who vote for union representation would jeopardize jobs. Tennessee Gov. Bill Lee on Monday called the union vote "a mistake" and "a loss for workers."

Ford Motor Co. CEO Jim Farley said in February that his company will "think carefully" about where it will build new vehicles after the UAW's strike last year. If the UAW organizes other automakers, it could raise their costs, leading them to build more vehicles outside the United States. But Su said President Joe Biden's administration is focused on supporting jobs in the country, noting grants to automakers to support a transition to electric vehicles.

"So the auto industry is an iconic American industry," Su said before speaking to the African American Mayors Association. "We want to make sure that employers who do right by their workers, who come to the bargaining table in good faith, who negotiate fair contracts, can also thrive and profit by using U.S. workers."

Biden is backing unions in other ways. Su noted the administration in January finalized a rule mandating unionized labor on all federal construction projects costing more than $35 million, despite complaints from nonunion contractors that the rule reduces competition and increases costs.

"That's one way that we ensure that you've got good union workers on jobs," Su said, saying union labor agreements are rising sharply on construction projects.

Southern states are also pushing laws that would claw back economic incentive dollars if companies recognize unions without requiring a secret ballot election. Every major southern auto plant has received state economic development assistance.

Federal law also allows employers to recognize unions if a majority of workers return signed cards authorizing unions to represent them, a process known as card check. Georgia Gov. Brian Kemp signed such a law Monday; Lee signed one in Tennessee last year. Alabama lawmakers are advancing such a measure.

Supporters believe unions can unfairly pressure workers into signing cards, while employers have a better chance of defeating unions in secret ballot elections. But those who support unions argue the laws violate the National Labor Relations Act, which allows voluntary recognition. They also say that employers use secret ballot elections to scare workers away from unions.

Su said she's not sure if the Labor Department will seek to challenge the laws, noting the National Labor Relations Board, which oversees union affairs, has primary responsibility. But she said that "there are federal standards beneath which no worker should have to live and work."

Su also decried union busting activity by employers, calling it "unacceptable." She cited a 2017 survey that showed nearly half of American workers would vote to unionize if they had the opportunity.

"This is part of President Biden's promise to center workers in the economy," Su said. "He has said he's the most pro-worker, pro-union president in history, and we are going to make good on that promise. And that includes making sure that workers have the right to join a union."