White House National Security Advisor Susan Rice, shakes hands with her incoming successor, Gen. Michael Flynn, during the 2017 Passing The Baton conference at the United States Institute of Peace in Washington on Jan. 10. (Mark Wilson/Getty Images)

President Obama’s National Security Council staff, the largest in history, has been cut by at least 15 percent over the past two years and “is smaller than the workforce of many think tanks,” Susan E. Rice, Obama’s national security adviser, said in a notice released Tuesday.

Acknowledging that “NSC staff reform remains a subject of debate in Washington,” Rice warned that the staff’s “size and structure will have a real-world impact on the advice given to the President, and whether the development and implementation of policy is properly coordinated across the U.S. Government.”

President-elect Donald Trump has said little about the organization of his White House staff. While he has made numerous appointments of senior advisers — one of whom, Monica Crowley, bowed out of an NSC job Monday amid allegations of plagiarism — it remains unclear how roles and responsibilities will be apportioned. Most of those named thus far have little or no experience in the executive branch, and several Cabinet nominees have expressed policy positions that don’t mesh with those enunciated by Trump.

Rice wrote that she had given her “candid and best advice” to her designated successor, retired Lt. Gen. Michael T. Flynn, about “how to shape an NSC staff” to address upcoming national security challenges. The notice was posted Tuesday on the White House’s website.

Each administration has made adjustments to the way its NSC staff is organized and operates, in most cases described in a presidential directive released shortly after inauguration. The national security adviser’s job traditionally is to chair meetings of principals — Cabinet and agency heads — and to coordinate among them to provide the president with a series of options for national security decisions.

“The buck still stops at the White House,” Rice wrote. “Especially in the face of polarized politics and a relentless, 24-hour news cycle. It falls to the NSC staff to frame for the President the toughest national security decisions, weighing disparate and sometimes competing information and equities from across our government.

“And it falls to the NSC staff to ensure that the President is fully briefed on the costs, trade-offs and risks associated with any major decision.”

Obama’s NSC staff has been widely criticized for its size — at least 400 people at its zenith, more than half of them policy advisers, with the rest in administration and technology support — and for what some of his former Cabinet officials have called excessive “micromanagement” of their departments.

“Over the last three decades,” Rice acknowledged, “the size of the NSC staff has grown in each presidential administration, Democratic and Republican alike. Until recently, this Administration’s NSC staff was no exception.”

The NSC, initially comprising the president and his top diplomatic and defense advisers, was established after World War II with a small White House secretariat. Over the years, the White House staff has expanded to cover virtually every area of national security policy. Numbering about 50 under President George H.W. Bush, it grew to about 100 in the Clinton administration and to more than 200 in the George W. Bush administration.

Rice attributed the unprecedented size under Obama to a merging of the NSC with the Homeland Security Council established after the 2001 terrorist attacks and “the intensification of emerging security challenges, from cyber threats to public health emergencies such as Ebola.”

“My strongly held view . . . is that the NSC staff works best when it is mean but lean,” she wrote in the notice. Rice, who became Obama’s third national security adviser in July 2013, said that she had reduced the overall staff size by 15 percent and cut policy and senior staff positions by 17 percent, to fewer than 180.

Much of the policy staff is not on the White House payroll but is on loan from other government agencies, including the intelligence community and the State and Defense departments.

In a speech last week at the U.S. Institute for Peace, Flynn thanked Rice for handoff advice and for “the initiation of various NSC reform measures that she has already undertaken,” without indicating additional steps that he intended to take.

“On the National Security Council we will serve four primary functions,” Flynn said. “We will advise the president on national security issues, we will formulate national security policy in coordination with the interagency process, we will monitor how policy is carried out, and we will also ensure that the president is properly prepared and staffed for the many national security-related events that we are likely to encounter.”

“And I would add one additional function. I want to add that President-elect Trump, we need to help him work with our partners in Congress on both sides of the aisle despite the difficulties that we will face.”