Last year, the foundation that runs the sprawling Virginia estate that was the home of President James Madison announced what it called a groundbreaking agreement to share power with a group representing descendants of the hundreds of enslaved workers who built and maintained the plantation.

The agreement, held up as a model for other historic sites of enslavement, promised to give the descendants equal authority on the foundation’s board, making them “coequals in sharing governing power and responsibility for the very site that enslaved their ancestors,” Gene Hickok, the board chairman, said in June 2021.

But after months of simmering conflict, the descendants’ group now accuses the majority-white board of thwarting that agreement to share control of Montpelier, the fourth president’s 2,650-acre estate, just outside Orange, Va.

A major breaking point came last month when the foundation’s board effectively stripped the descendants’ group, the Montpelier Descendants Committee, of the exclusive power to recommend representatives to serve on the board, and affirmed its own power to nominate those members.

In a statement issued by the foundation, Mr. Hickock said the goal was to broaden the pool of applicants beyond those favored by the committee.

“Currently, there are five descendants of enslaved persons on the 16-member board,” he said. “Three are named by the M.D.C., and two are named by the foundation. We are working toward a board makeup that is 50 percent from the descendants’ community. That is very much on track, despite earlier misinformation and inaccurate reporting.”

But James French, the chairman of the Montpelier Descendants Committee and a foundation board member, said that the change effectively gave the majority-white board the power to choose which descendants would help govern the plantation — a shift, he said, that flew in the face of equal governance and was “by definition racist.”

“It was merely a power grab, and they’re trying to maintain and get credit in the headlines for sharing power when they never intended to share power,” Mr. French said. “We’re just hearing all of this gaslighting and delays — week after week and month after month — and, in the meantime, the world still thinks they’ve accepted structural parity, which they haven’t.”

Tensions came to a head this week when the foundation fired three staff members, including an executive vice president, prompting a rebuke by the National Trust for Historic Preservation, which owns the property.

“The National Trust has been working to achieve a resolution to the very public dispute between the foundation and the Montpelier Descendants Committee, but these and other recent actions by the foundation lead us to question whether a resolution is possible under the current leadership,” the trust said in a statement.

Mr. Hickock said in a statement that the “actions taken this week were for repeated and disruptive violations of our employment policies.” He said the issues involved dated back more than 18 months and “have to do with employees ignoring or violating the policies of the foundation and continually seeking to undermine the management of the foundation.”

“The atmosphere at Montpelier had become untenable and toxic, aggravated by misleading public statements made by the M.D.C. and by bias demonstrated by the National Trust for Historic Preservation,” Mr. Hickock said. “Work was not getting done. Projects were being halted. Montpelier’s leadership could not allow that to continue.”

Matt Reeves, who was fired as the director of archaeology and landscape restoration after 22 years at Montpelier, rejected that characterization.

“All of this is related to the board failing to recognize the partnership we’ve had with the descendant community for 20 years,” Mr. Reeves said in an interview.

Montpelier had worked with descendants to present a more accurate picture of Madison and to reconcile his legacy as a slaveholder with his reputation as a founding father. A 2017 exhibition called “The Mere Distinction of Colour,” for example, sought to illuminate the lives of enslaved workers at Montpelier and the political and economic factors that cemented slavery in the Constitution.

“It’s a major site of slavery and enslavement in the United States, and a major site of the birth of freedom in the United States, and those two ideas coexist in the United States, and they need to be explored in a completely new way,” Mr. French said. “Both of those paradigms speak not just to the past, but to the present.”

In an apparent attempt at brokering an agreement with the Montpelier Descendants Committee, the foundation said on Wednesday that it would accept nine new board candidates put forward by the committee. The new board members would be elected in May and seated in two groups, in July and October.

Roy F. Young II, the president and chief executive of the Montpelier Foundation, said the board members’ terms would be staggered to avoid a mass vacancy in the future.

“It has been difficult on everyone, but it is important work to do,” Mr. Young said. “I am positive that we will find a way forward and create true structural parity with descendants and the foundation.”

But Greg Werkheiser, a lawyer for the Montpelier Descendants Committee, called the proposal “more trickery,” adding that the decision not to seat the new members immediately would allow the foundation to maintain control for the next five months, a power he said it would use to prevent the rehiring of fired staff members and to retaliate against others.

“All that has been lost at Montpelier and more can be regained,” Mr. Werkheiser said. “But it will require new leadership, and white power is not easily relinquished.”