The appointment of Neera Tanden to head the White House Office of Management and Budget was thrown in doubt Friday as Sen. Joe Manchin of West Virginia became the first Democratic lawmaker to oppose her confirmation.

During her confirmation hearings, Tanden apologised for spending years bullying top Republicans on social media. She is a former advisor to Hillary Clinton and served as president of the liberal-leaning Center for American Progress. With the Senate equally split between 50 Republicans and 50 Democrats, she’ll likely need support from at least one Republican to gain approval.

“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 in a statement. He went on to say that, 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.”

It’s the first real challenge that Biden has faced on an appointment, with most of his choices for Cabinet positions passing through the chamber with bipartisan support. Tanden had also disparaged several Democrats on social media, most notably Sen. Bernie Sanders, the independent from Vermont.

Biden, asked Friday if he would pull Tanden’s nomination, said he wouldn’t.

“I think we are going to find the votes and get her confirmed,” Biden said.

Moments earlier, the White House had released a statement defending her.

“Neera Tanden is an accomplished policy expert who would be an excellent Budget Director and we look forward to the committee votes next week and to continuing to work toward her confirmation through engagement with both parties,” White House press secretary Jen Psaki said.

Tanden will be the first woman of colour to head the OMB, which directs efforts to ensure an administration’s goals are expressed in legislation and regulations.

The Senate Budget Committee is expected to vote on her appointment next week.