West Virginia Democratic Sen. Joe Manchin III said Friday he would oppose President Joe Biden’s choice for White House budget director, in a decision that could imperil Neera Tanden’s confirmation in a 50-50 Senate.

The maverick senator, who sometimes strays from his party and is considered a pivotal swing vote, said he could not support Tanden for director of the Office of Management and Budget because of her history of making incendiary comments about Republican senators and Senate Budget Chairman Bernie Sanders, I-Vt.

In a statement, Manchin cited past comments and tweets “personally directed towards my colleagues on both sides of the aisle,” including Sanders and Minority Leader Mitch McConnell, among others.

“I believe her overtly partisan statements will have a toxic and detrimental impact on the important working relationship between members of Congress and the next director of the Office of Management and Budget,” Manchin said. “For this reason, I cannot support her nomination.”

Tanden, a former policy aide in the Clinton and Obama administrations, has thrown some sharp elbows as president of the left-leaning Center for American Progress. She issued tweets attacking Senate Minority Leader Mitch McConnell as “Voldemort” and “Moscow Mitch” and referred to Maine Sen. Susan Collins as “the worst.”

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Tanden also sparred with Sanders when he sought the 2016 Democratic presidential nomination against Hillary Clinton. At a nomination hearing this month, Sanders scolded Tanden for “vicious attacks made against progressives, people I have worked with and me personally.”

Sanders declined to say Friday whether he would vote to confirm Tanden.

 “I worry less about what Ms. Tanden did in the past than what she’s going to do in the future,” he told CNN. “I will be speaking to her early next week to get a sense of what she wants to do as head of the OMB.”

Biden told reporters Friday he’s sticking with his nominee for OMB chief. And White House Press Secretary Jen Psaki issued a statement defending Tanden as “an accomplished policy expert who would be an excellent budget director,” adding that the administration will continue “to work toward her confirmation through engagement with both parties.”

Tanden repeatedly apologized for her tweets during her confirmation hearings, many of which she had deleted in seeking the White House job. And she pledged that her use of social media would be “radically different” if she is confirmed by the Senate.

“I recognize it’s really important for me to demonstrate that I can work with others and I look forward to taking that burden,” she said at her Budget Committee confirmation hearing.

But Manchin said he had no confidence that Tanden could become an effective budget director given her history of demeaning senators.

“As I have said before, we must take meaningful steps to end the political division and dysfunction that pervades our politics,” Manchin said in his statement. “At a time of grave crisis, it is more important than ever that we chart a new bipartisan course that helps address the many serious challenges facing our nation.”

His decision to oppose Tanden puts her confirmation in jeopardy. Republicans have already expressed misgivings and Sanders declined to offer an endorsement during the Budget hearing. If Republicans are united in opposing Tanden, Manchin’s vote would quash her confirmation.

“In a time of unity, we’re picking someone who throws sharp elbows and there’s going to be a consequence for that, hopefully on our side,” said South Carolina Sen. Lindsey Graham, the Budget panel’s top Republican, at Tanden’s nomination hearing.

The two Senate panels with jurisdiction over OMB nominees, Budget and Homeland Security and Governmental Affairs, have scheduled votes on Tanden for next Wednesday. Under Senate rules, only one panel is required to report her nomination to the floor; the other is automatically discharged after 30 days if it hasn’t acted.

Manchin isn’t a member of either panel that’s scheduled to meet next week, but his vote will be critical if Tanden’s nomination reaches the floor.