Senate Republicans are expressing frustration with the Biden administration over its decision to add Acting Secretary of Labor Julie Su — whose nomination to the role was never confirmed — to the presidential line of succession.

In a Tuesday letter to President Biden, Alabama GOP Sen. Katie Britt and 29 of her Republican Senate colleagues expressed "grave concerns" and requested clarification from the administration for its "apparent belief" that Su is "eligible to assume the office of President of the United States pursuant to the presidential line of succession as established by Congress in the Presidential Succession Act of 1947."

"As you are well aware, since March 14, 2023, the United States Senate has declined to confirm Ms. Su’s nomination as Secretary of Labor, and she continues to lack adequate support from both Republican and Democratic members of the Senate to be confirmed to that position," the senators wrote. "Despite that reality and Ms. Su becoming the longest-ever Cabinet nominee to await confirmation in a time when the same party controls the White House and the Senate, the White House has chosen to keep her in place as Acting Secretary of Labor on an indefinite basis and has also listed her on the White House website as a member of the Cabinet ‘[i]n order of succession to the Presidency.’"

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Julie Su, right, with President Joe Biden raising up her hand

President Biden links arms with Julie Su, his nominee to be the next secretary of labor during an event in the East Room of the White House in Washington, D.C., on March 1, 2023. (Win McNamee/Getty Images)

Listed ninth in the "order of succession" to the presidency on the White House's website, Su previously served as California's labor secretary under Gov. Gavin Newsom from 2019 to 2021.

"While you and your Administration have clearly decided to ignore congressional intent in keeping Ms. Su in place in her current role, it would be unfortunate if you have decided to further discount congressional intent — and violate the law — by taking the position that Ms. Su is eligible for placement in the presidential line of succession," the Republican senators added.

Pointing to the 25th Amendment to the U.S. Constitution, the senators reminded President Biden that Congress has "the power to set the presidential line of succession beyond the Vice President."

"As Ms. Su has failed to be ‘appointed, by and with the advice and consent of the Senate’ as Secretary of Labor, we strongly urge the White House to clarify its position and views regarding Ms. Su’s eligibility for the presidential line of succession and, in the event it was to become necessary, to assume the presidency," they wrote. "It is unimaginable to think that this Administration believes someone who has neither been duly elected nor confirmed by the Senate to the position of Secretary of Labor could be President of the United States. Suggesting that Ms. Su is eligible to be in the presidential line of succession is antithetical to our system of governance and the bedrock principles on which our Republic rests."

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"The Biden Administration continues to attempt to rule by unilateral decree rather than govern with the advice and consent of Congress. Ms. Su doesn’t even have adequate support from members of her own party in the Senate to be confirmed as Secretary of Labor," Britt told Fox News Digital. "In sum, the Biden Administration is now seemingly asserting both that it can freely ignore the will of the Senate by keeping Ms. Su in place indefinitely as Acting Secretary, and that Ms. Su could instantaneously ascend from her current role to serve as President of the United States."

Sen. Katie Britt, Alabama Republican

"Ms. Su doesn’t even have adequate support from members of her own party in the Senate to be confirmed as Secretary of Labor," Alabama GOP Sen. Katie Britt told Fox News Digital. (Anna Moneymaker/Getty Images)

"President Biden should immediately clarify whether he believes someone who has neither been elected by the people nor approved by the people’s elected representatives as a member of the Cabinet could assume the presidency," she added.

Additionally, the senators once again called on Biden to "withdraw Ms. Su’s nomination and put forward a nominee for Secretary of Labor who is capable of garnering sufficient support on a bipartisan basis to be confirmed."

Su's nomination to the role of labor secretary was met with immense backlash, both from Republicans and Democrats, earlier this year. As a result, she was never confirmed to the role and has remained as the acting secretary of labor ever since.

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GOP senators accused Su of stonewalling their requests for information on her support for more regulations on gig work in California and a memo sent during her tenure as the state's top labor official that instructed state employees not to cooperate with ICE officials looking for undocumented migrants.

"She has avoided answering questions whenever possible and she has refrained from providing distinct specificity to her answers when she has responded to inquiries," Sen. Mike Braun, R-Ind., and other Republicans wrote in a letter to Biden about Su's nomination in June. "Given this present state of affairs, we respectfully urge you to withdraw the nomination."

acting Labor Secretary Julie Su testifying to Congress

Acting Labor Secretary Julie Su speaks during a House Workforce and Education Committee hearing in Washington, D.C., on June 7, 2023. (Eric Lee/Bloomberg via Getty Images)

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The nomination of Su, who has been serving as acting secretary since the resignation of Marty Walsh in March, was narrowly approved by the Senate Health, Education, Labor and Pensions Committee in April. However, Su has not received a vote by the full Senate, where it is unlikely Su would be confirmed to the post.

In addition to Britt, the letter from Senate Republicans to Biden was signed by Senate Minority Leader Mitch McConnell, R-Ky.; Senate GOP Whip John Thune, R-S.D.; Senate GOP Conference Chair John Barrasso, R-Wyo.; Republican Policy Committee Chair Joni Ernst, R-Iowa; and Vice Chair of the Senate Republican Conference Shelley Moore Capito, R-W.Va., among others.