Treasury Secretary Steven Mnuchin, right, listens as President Trump speaks during a meeting on the federal budget in the White House on Feb. 22. (AP Photo/Evan Vucci)

President Trump has promised a major rewrite of the U.S. tax system, and when he released an outline of his plan last week, his top officials promised the administration would fight to make that vision a reality.

Trump's administration, however, has yet to pick the officials who would be chiefly responsible for negotiating a tax deal with lawmakers on Trump's behalf.

In particular, the president has not put forward any names for the post of assistant secretary for tax policy, an office that was instrumental for presidents Ronald Reagan and George W. Bush as they successfully passed tax cuts early in their administrations.

The vacant offices of the deputy assistant secretaries for tax analysis and international tax affairs could also prove to be obstacles for Trump as he seeks to move major tax legislation through Congress.

“It helps a lot to have somebody whose full-time job is looking at the tax issues and not everything else that’s going on in the world,” said Pam Olson, who held that post in Bush's administration from 2002 to 2004.

That assessment, shared by lobbyists and tax policy experts, is at odds with the assurances given by Treasury Secretary Steven T. Mnuchin and other administration officials, who have frequently pointed out that the administration has several dozen career staffers working on proposals in the Treasury Department's Office of Tax Analysis.

“We have over 100 people in the tax department, and we’ve been running numbers, you know, for several months,” Mnuchin said Monday at a conference in Beverly Hills.

Many of them have spent their professional lives studying the system and can answer any questions Trump might have about taxes. Yet they might not be able to make much progress on the details of a plan without clear marching orders from Trump's own people, according to Olson and other former Treasury officials.

“The administration would be in a stronger position if they had some of the roles in between the secretary and the career staff filled,” said Olson, now at PricewaterhouseCoopers.

Throughout the entire federal government, Trump has put forward just 37 names for 530 vacant posts requiring confirmation by the Senate, The Washington Post reported last week, causing frustration among members of his Cabinet. The vacancies at the Treasury Department suggest how the White House's unusually slow process of vetting candidates could impede Trump's progress on one of his main domestic objectives.

The assistant secretary could serve as Trump's “tax policy whisperer,” in the words of Mark Mazur, who held the post under President Barack Obama. “Secretary Mnuchin’s got, like, a million things on a his plate, tax reform being one of them,” Mazur said. “I don’t think he’s going to every meeting.”

“The White House can do a wonderful job of putting together the framework at 10,000 feet,” said Dean Zerbe, the Senate Finance Committee's senior counsel under Sen. Charles E. Grassley (R-Iowa.). “Fleshing out ideas — putting meat on the bone if you will — is what the assistant secretary for tax and his team, or her team, can really bring to the table.”

The White House did not respond to a request for comment.

Bush moved swiftly to fill the position. On March 1, 2001 — barely five weeks after his inauguration -- the Senate confirmed his first assistant secretary of tax policy, Mark Weinberger. Zerbe said there was "no question" that Weinberger's leadership helped Bush succeed in cutting taxes during his first year in office.

Reagan elevated Norman B. Ture, who held the post in his administration, to the rank of undersecretary. A New York Times story at the time credited Ture as the “architect” and “principal designer” of Reagan's tax cuts.

Trump does have other subordinates who can help him achieve his goals on taxes. Shahira Knight, a special assistant to the president, is widely respected as an expert on the system.

Additionally, the president has named Kevin Hassett, a conservative economist who has advised Bush and former Republican presidential nominee Mitt Romney, to chair his Council of Economic Advisers. In his academic research, Hassett has investigated the question of how corporate taxes affect workers' pay and the overall economy.

“You have to deal with the realities you have,” said Michael Mundaca, who served as assistant secretary for tax policy under Obama. “The administration will make do with the team that it has, and I think it has the team to be able to move this ahead.”

It will take several weeks, at a minimum, for the Senate to confirm any nominees that Trump puts forward for these posts. The administration might not have that much time, Mundaca said, given the urgency of moving legislation before lawmakers start returning to their districts next year to prepare for the midterm elections.

On the other hand, a confirmation would give the nominee an opportunity to meet with senators and their staffs and talk over Trump's ideas on taxes. Getting to know one another could be useful both for Congress and for the administration, Mazur argued.

“Those kinds of relationships matter when you’re trying to get legislation enacted, especially if it’s difficult,” he said. “It’s important for an administration to be showing progress on this, and to really have, like, capable people able to discuss the details of tax policy with the members and staff.”