WASHINGTON (AP) — President Joe Biden's nomination of Neera Tanden to lead the White House Office of Management and Budget was thrown further into doubt on Monday as moderate Republican Sens. Susan Collins of Maine and Mitt Romney of Utah said they would vote against confirming her.

On Friday, Sen. Joe Manchin of West Virginia became the first Democratic lawmaker to oppose the confirmation of Tanden, who would be the first woman of color to lead the agency. With doubts growing about Tanden's chances for confirmation, the White House called her "an accomplished policy expert," and Biden said he was sticking with her.

Collins, though, said Monday that Tanden has "neither the experience nor the temperament to lead this critical agency," which heads efforts to ensure an administration's priorities are reflected in legislation and regulations. Collins blamed Tanden's past actions and said they "demonstrated exactly the kind of animosity that President Biden has pledged to transcend."

Romney will also oppose Tanden, a spokesperson confirmed, because of her rhetoric on social media.

During her confirmation hearings, Tanden apologized for her prolific attacks against top Republicans on social media. Tanden is a former adviser to Hillary Clinton and served as president of the liberal-leaning Center for American Progress. With the Senate evenly divided between 50 Republicans and 50 Democrats, and with Vice President Kamala Harris serving as a tiebreaking vote, losing Manchin means Tanden would need support from at least one Republican to win confirmation. Tanden really can't afford to lose another Democratic vote.

The Senate Budget Committee is scheduled to vote on Tanden's nomination this week. It's the first real test that Biden has faced on a nomination, with most of his picks for Cabinet positions sailing through the chamber with bipartisan support.

Collins criticized Tanden for deleting tweets in the days before her nomination was announced and said that "raises concerns about her commitment to transparency." She said Congress "has to be able to trust the OMB director to make countless decisions in an impartial manner, carrying out the letter of the law and congressional intent."

"The OMB needs steady, experienced, responsive leadership," Collins said in a statement. "I will vote against confirming Ms. Tanden."

Manchin said bipartisanship is "more important than ever" as the nation faces many crises and suggested Tanden was overtly partisan.

"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.

Tanden had also disparaged some Democrats on social media, most notably Sen. Bernie Sanders, the independent from Vermont.

Tanden acknowledged spending "many months" removing past Twitter posts, saying, "I deleted tweets because I regretted them." She said she regretted some of her "past language" on social media, but she refused to say she deleted the comments to help her nomination.

Republicans have signaled that Tanden's confirmation process may trigger a political battle unseen with other Biden nominees, given her history of criticism of GOP lawmakers she'd now have to work with.

Republican senators have griped about Tanden's "harsh criticism" and "personal attacks" in her tweets, such as calling Sen. Tom Cotton of Arkansas "a fraud" and saying "vampires have more heart" than Sen. Ted Cruz of Texas.

The Republican senators' complaints about Tanden's tweets, though, seemed to ignore the hypocrisy of criticizing her for her social media content after spending four years largely failing to condemn the toxic tweets of former President Donald Trump, who was recently banned from Twitter.

Trump called a woman " Horseface," made racist attacks against Democrats, lied about the result of the 2020 presidential election and last month finally had his Twitter account revoked, with the social media platform citing "the risk of further incitement of violence" following the deadly insurrection at the U.S. Capitol days earlier.

Biden, asked Friday whether 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.

Tanden, 50, has said her values reflect personal experience with the government programs she would play a key role in supporting. The child of Indian immigrants, her parents divorced when she was 5, and her mother spent two years on government support, food stamps and Section 8 housing vouchers in the suburbs of Boston before eventually getting a job as a travel agent.

Tanden has "lived experience having benefitted from a number of federal programs as a kid," the White House said on Monday.

"Looking ahead to the committee votes this week and continuing to work toward her confirmation," White House press secretary Jen Psaki tweeted.