West Virginia Sen. Joe Manchin said Thursday he will oppose Julie Su’s nomination as Secretary of Labor, dealing a major blow to the Californian’s prospects of getting confirmation.

Manchin announced his position on Thursday afternoon after months of refusing to commit to a vote. Senator Mark Kelly (D-Ariz.) Politico told Thursday that he would support Su’s bid, while Sen. Kyrsten Sinema (D-Ariz.) and Jon Tester (D-Montana) have yet to take a position.

If Sinema or Tester oppose Su, her nomination will likely die in the Senate, a setback for the influential worker and a dramatic blow to the White House, which invested significant time and resources in its bid.

Any labor secretary should demonstrate his ability to “lead both labor and industry to reach compromises that are acceptable to both parties,” Manchin said in a statement. “While her credentials and qualifications are impressive, I am genuinely concerned that Julie Su’s more progressive background prevents her from doing so and therefore I cannot support her nomination as Labor Minister,” he wrote.

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who currently serves as Acting Secretary of Labor,

would be Biden’s first Asian-American cabinet secretary.

In the weeks leading up to Manchin’s announcement, her supporters were more optimistic about her chances. Su traveled to the west coast and helped broker a deal between

harbor workers

dockers and their

administrators

employers. White House officials announced the deal as proof of Su’s abilities, and continued to push her nomination even without a scheduled vote. The White House even gathered a daily “war room” to discuss her nomination.

The White House did not immediately respond to a request for comment.

But Su’s track record in California has sparked some concern in Washington. She was a key enforcer of AB 5, a controversial gig worker law, and opponents of her nomination feared that Su would take similar policies to the federal level, despite saying she would not.

A coalition of companies, freelancers and other workers called Stand Against Su has also funded advertisements that oppose her. The ads targeted Montana and West Virginia, where Manchin and Tester are up for re-election. Members of Congress have pushed Su to her California plate as she continues to fill the role of secretary of labor in an acting capacity.

That’s something unique to me, Senator Mazie Hirono, a longtime supporter of Su, told The Times last month. A lot of money has come out and billboards have been put up in certain states to encourage those senators to think twice before voting for her.

No federal law dictates when Su should leave her acting position

if she doesn’t get confirmation

. Senator Bernie Sanders told NBC News that if she doesn’t have the votes, “she should stay where she is.”