WASHINGTON — Democratic Sen. Joe Manchin does not plan to endorse President Joe Biden’s nominee for Secretary of Labor and has requested a list of alternative candidates, according to two sources with knowledge of the matter.

Manchin’s opposition to candidate Julie Su could prove fatal if another senator meeting with the Democrats also opposes her.

After Manchin, DW.Va., met with Su last week, he joined a call with the International Franchise Association, which opposes Su’s nomination because of his support for policies the organization says , “would dismantle the franchise business model.”

«IFA has suggested various alternatives to … His in the mold of former secretary Marty Walsh, who has received bipartisan support and rallied the parties,» an IFA spokesperson told NBC News in an emailed statement.

Su currently serves as acting secretary of labor and was nominated by Biden to succeed Marty Walsh, who resigned earlier this year to lead the National Hockey League Players Association. In 2021, she was confirmed by a three-vote margin as Assistant Secretary of Labor in the face of unanimous Republican opposition.

The IFA spokesman also said the group provided alternatives to «several senators», several of whom have yet to publicly disclose their intentions on Su.

Democratic-turned-independent Sen. Kyrsten Sinema of Arizona was on the same call with Manchin, according to two sources.

Manchin met with Su for nearly 45 minutes on May 3, later telling reporters that he was «very nice» and «respectful» but had not yet decided how he planned to vote on her nomination. When asked if he had a timeline for when he would make a decision, Manchin said no.

“There are still some, there are still some things that I need to review,” Manchin said at the time. “We are going through some things. We had a good dialogue.»

Manchin typically looks for alternatives and presents them to Senate leaders when announcing his opposition to a candidate, a source told NBC News.

It is unclear at this time if Manchin will announce his intention not to support Su. A source with knowledge of the matter told NBC News that Manchin had yet to state his position because he was waiting for Sen. Dianne Feinstein, D-Calif., to return to work after suffering health problems. Feinstein is expected to endorse hers.

Manchin is also looking forward to the possibility of Sinema coming out in support of Su, the source added.

When contacted for comment, a Manchin spokesperson said the senator «has not announced his position on this nomination and will not comment on the rumours.»

A Sinema spokesman did not say whether he plans to support Su.

Senate Majority Leader Chuck Schumer, DN.Y., has not said publicly when a vote on Su’s nomination will take place on the full Senate, and senators are not scheduled to return to session until after the Memorial Day.

Schumer’s office did not immediately respond to a request for comment late Friday.

In the closely divided Senate, if Manchin were to vote against Su, just one more defection from his side of the aisle could ruin his nomination. It’s unclear where other moderate Democrats like Sens. Mark Kelly of Arizona and Jon Tester of Montana, as well as Sen. Angus King, I-Maine, stand on Su’s nomination.

Manchin, Sinema, King, Kelly and Tester endorsed her for the alternate role in 2021.

Outside of Washington, trade groups in West Virginia and Arizona, Manchin’s, Kelly’s and Sinema’s home states, sent letters to their senators urging them to oppose Su just before his April confirmation hearing before the Senate Committee on Health, Education , Work and Pensions.

On Friday night, a White House official told NBC News that «Su is continuing the work that she and Secretary Walsh started and we are continuing to push for [her] Confirmation of the Senate.