WASHINGTON (AP) — President Joe Biden’s Cabinet is taking shape at the slowest pace of any in modern history, with just over a dozen nominees for top posts confirmed more than a month into his tenure.

Among Biden’s 23 nominees with Cabinet rank, just 13 have been confirmed by the Senate, or a little over half. And among the 15 core nominees to lead federal agencies, 10 have been confirmed, or about two thirds. According to the Center for Presidential Transition, about a month into their first terms, the previous four presidents had 84% of their core Cabinet picks confirmed.

On Tuesday, Biden's Cabinet was thrown into further uncertainty when his nominee to lead the White House budget office, Neera Tanden, withdrew from consideration after her nomination faced opposition from key senators on both sides of the aisle.

The delay in confirmations means some departments are left without their top decision-makers as they attempt to put in place policies to address the overlapping crises brought on by the coronavirus pandemic.

Former Health and Human Services Secretary Donna Shalala said there are a number of “big decisions” at HHS and across the federal government that are waiting on leadership from the top.

“It’s very unfortunate. And in the middle of a huge health crisis, it’s the wrong thing to do,” she said. “Civil servants are capable, but they need leadership. And they’re used to having leaders.”

Shalala was confirmed two days after President Bill Clinton was sworn in, and said she had her chain of command ready to go and could immediately dig into a long list of decisions and policy changes.

California Attorney General Xavier Becerra, the Biden administration’s HHS nominee, will get a committee vote Wednesday, and he’s expected to receive easy confirmation. But Shalala pointed to a laundry list of issues — from oversight of hospitals, health care companies and nursing homes during the pandemic to issues surrounding drug pricing, telemedicine and child care services — that urgently need his input.

Lacking a department head, she said, “just slows everything down.”

Matt Stier, president and CEO of the Partnership for Public Service, a nonprofit organization that tracks presidential transitions, said federal departments tend to act more conservatively around decision-making and shifting policies without the top brass in place.

“Missing the top person means that it’s pretty difficult to actually address the very big questions and to make big changes," he said. “And there’s a natural conservatism in place when people don’t know yet what the top person is going to really want.”

The slow pace in confirmations partly results from the delay in the transition process resulting from President Donald Trump's attempts to dispute his loss in the 2020 presidential race and from what the Biden White House says was a lack of cooperation from Trump administration officials.

Senate Democrats did not win a majority of seats in the chamber until the Jan. 5 Georgia runoff elections, and then it took nearly a month for Democratic and Republican leadership to agree on a resolution governing the organization of the upper chamber, which further delayed committee work.

And Democrats privately acknowledge that Trump’s second impeachment trial also slowed down the process some, eating up a week of valuable time in the Senate and bogging lawmakers down with other work beyond reviewing and processing Biden’s nominees.

Still, Biden transition spokesman Andrew Bates said that after the delays “stemming from the previous administration’s resistance to the will of the American people,” the relatively smooth confirmation progress in recent weeks “is both welcome and appreciated.”

He added, however, “it is hardly enough, and nominees with strong bipartisan support — and who are critical to defeating the pandemic and turning our economy around with the creation of millions of jobs — remain needlessly obstructed by individual members. That must change.”

The Biden administration has prioritized confirming those nominees who are key to national security, the economy and public health decisions. Biden does have in place his director of national intelligence, and his top brass at the departments of State, Homeland Security and Defense, as well as his treasury secretary.

But in addition to waiting on Becerra at HHS, the administration lacks top leaders at the Justice Department, Housing and Urban Development and the Small Business Administration, departments that will be key to some of Biden's top priorities and the implementation of his $1.9 trillion coronavirus aid bill, if it's passed into law later this month.

And the delay in confirming top posts also means a delay in confirming and seating deputy secretaries and undersecretaries, who are often in charge of the nitty gritty in implementing major policy. Shalala noted, for instance, the Centers for Medicare & Medicaid Services will offer guidance on how insurers should cover coronavirus costs and implementation on aspects of the COVID-19 aid bill, and currently only has an acting administrator. She also noted HHS has deputies who oversee everything from refugee resettlement to child care programs.

And Tanden's withdrawal Tuesday raises further questions about the Biden administration's budget process.

The White House has yet to offer a timeline for releasing its budget, citing the transition delays and a lack of cooperation from the Trump administration. That puts them behind most recent presidents, who typically submit written budget toplines to Congress by the end of February, though Trump didn’t submit his until mid-March.

The Biden administration has not been completely hamstrung by the slow pace of confirmations, however. The White House has issued a number of executive orders outlining policy reviews and changes that are underway at federal departments, and civil servants are working through key policy decisions, even without Senate-confirmed leadership in place.

For instance, while Biden’s nominee to head the Department of Education, Miguel Cardona, was just confirmed by the Senate on Tuesday, the department's acting head last month put out guidelines requiring states to administer standardized tests despite the pandemic.

And Stier noted that the Biden administration has installed hundreds of non-Senate-confirmed staff across the federal government, helping to provide guidance even without department heads in place. Biden himself swore in more than 1,100 non-Senate-confirmed staff throughout the federal government on the first day of his presidency, a number Stier said was unprecedented.

"It ameliorates the problem in that you then have in place people who can provide guidance to the career team about what the administration’s positions and priorities are," Stier said.

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Associated Press writers Ricardo Alonso-Zaldivar, Matthew Daly, Collin Binkley and Ashraf Khalil contributed to this report.