The Nation of Islam faces a leadership vacuum and possible disintegration when the bow-tied black separatist Louis Farrakhan, 73, gives his final public address on Sunday in Detroit.

Mr. Farrakhan’s stewardship of the Nation of Islam, a religious and political movement founded in Detroit in 1930 that has been accused of promoting racism and anti-Semitism, is drawing to a close after nearly three decades. He has been suffering from prostate cancer and recently underwent 12 hours of major abdominal surgery.

Although Mr. Farrakhan has billed his upcoming speech at Ford Field — a 65,000-seat stadium in Detroit — as his last major public engagement and is expected to retreat from political life, he has made little effort to arrange his succession. Movement clergy insist that no leader has been groomed to take over in the event of Mr. Farrakhan’s death.

Citing his “health condition,” Mr. Farrakhan transferred control of movement operations to an eight-member executive board that he appointed in September. “In this period of testing, you can prove that the Nation of Islam, a people submitting to the Will of Allah, is more than the physical presence of any individual, and that will live long after I and we have gone, because it is rooted in an idea that comes directly from Allah,” he wrote in a letter to followers.

Kevin Muhammad, who is the minister of the Nation of Islam’s Muhammad Mosque 7 in Harlem — a position held in the 1960s by black militant leader Malcolm X and later by Mr. Farrakhan — surmised that in Mr. Farrakhan’s absence, the movement would be led by the board and not by any one person. “One day, the minister will leave us, and when he does, we don’t need a leader,” he told The New York Sun. “No one can be like the Honorable Minister Farrakhan is to us.”

At the height of its popularity in the 1970s, the Nation of Islam attracted hundreds of thousands of adherents and enjoyed widespread influence, some who study the movement said. In 1995, Mr. Farrakhan staged his most impressive event, the Million Man March on Washington, which was intended to encourage black men to become more closely involved in politics, community, and family life.

But in recent years, the Nation of Islam has lost a great deal of its momentum as Mr. Farrakhan’s health has declined and as black Muslims in America have embraced other Islamic denominations. Some black leaders have also carefully distanced themselves from Mr. Farrakhan, who repeatedly has made anti-Jewish statements, among them calling Judaism a “dirty religion” and saying he believes America is run by a Jewish cabal.

Some who study the Nation of Islam say the notion of “black nationalism” has lost much of its political attraction at a time when a black American, Senator Obama, is a front-runner for the Democratic presidential nomination.

“The Nation of Islam is outdated and outvoted,” the author of “Prophet of Rage: A Life of Louis Farrakhan and His Nation” (HarperCollins, 1997), Arthur Magida, said. Even if the Nation of Islam remains unified after Mr. Farrakhan’s death, “its leader wouldn’t be able to call a Million Man March,” he said.

Mr. Magida estimates that there are about 12,500 Nation of Islam members nationwide, and a professor of religion and Africana studies at Vassar College, Lawrence Mamiya, said the Nation has 50,000 members throughout the world, including those in prisons. The movement’s leaders would not release or speculate on membership numbers.

There is speculation that in his Sunday address, titled “One Nation Under God,” Mr. Farrakhan will try to bring about a rapprochement between the Nation of Islam and mainstream Islam, both of which derive some traditions and theology from the Koran.

Unlike other Islamic denominations, the Nation of Islam’s doctrine holds that Allah appeared incarnate in Detroit in 1930 and that the black race began 66 trillion years ago after a massive explosion separated the Earth and the moon. According to Nation of Islam philosophy, white people appeared on the planet just 7,000 years ago — their presence the result of an experiment by an evil magician named Yakub.

The Sunni Muslim imam of Masjid Al-Taqwa in Brooklyn, Siraj Wahhaj, led Friday prayers at the Nation of Islam’s Saviour’s Day conference, which began yesterday. “That would never have happened a few years ago,” the director of the Islamic World Studies program at Chicago’s De Paul University, Aminah McCloud, said.

A spokesman for the Council on American-Islamic Relations, Ibrahim Hooper, said the inclusion of Imam Wahhaj could signal Mr. Farrakhan’s attempt to reach out to other Islamic denominations “Over the years, he’s put out feelers to the mainstream Muslim community,” Mr. Hooper said. “Now, apparently as he’s nearing the end of his life, he may want to make those tentative moves more permanent.”

Elijah Muhammad, who led the Nation of Islam for more than 40 years, appointed as successor his son W.D. Muhammad, who eschewed black separatism in favor of Sunni Islam and American patriotism. Mr. Farrakhan severed ties with W.D. Muhammad in 1978 and restarted the Nation of Islam.

Some have predicted that Ishmael Muhammad, another son of Elijah Muhammad and the assistant minister at the Nation of Islam’s Mosque Maryam headquarters in Chicago, could lead a post-Farrakhan Nation of Islam.

But Ishmael Muhammad told the Sun that he does not believe he will be asked to succeed Mr. Farrakhan soon. “There has never been anyone like him, and there will never again will be,” he said of Mr. Farrakhan. “Whoever God is grooming and preparing will manifest and emerge at the proper time. For the foreseeable future, our leader is Brother Farrakhan.”