The reshuffle at the highest echelons of the Bush administration accelerated yesterday as a prominent New Yorker won nomination to a key Cabinet post, while America’s top diplomat at the United Nations announced his departure.

President Bush will nominate a former New York police commissioner, Bernard Kerik, to head the Department of Homeland Security, an aide to Mr. Bush confirmed last night. The formal announcement is expected in Washington today.

Meanwhile, the American envoy to the United Nations, John Danforth, resigned yesterday, less than six months after being confirmed to the post.

If confirmed by the Senate, Mr. Kerik, 49, would replace the nation’s first secretary of homeland security, Tom Ridge, who announced his resignation just three days ago.

A White House official who asked not to be identified said Mr. Bush was impressed with Mr. Kerik’s role at the helm of the New York Police Department during the September 11, 2001, terrorist strikes.

“Commissioner Kerik is a proven crisis manager with credibility and a firsthand understanding of the war on terror,” the Bush aide said. “The nation already knows of his service in response to the World Trade Center attack of September 11. He was on the scene within minutes. He helped coordinate on the ground the rescue and recovery efforts.”

Political leaders from the area hailed Mr. Kerik’s nomination and said it ensures that New York will remain at the center of the nation’s anti-terror campaign.

“If there were ever a state that deserved to have one of its citizens appointed head of homeland security, it’s New York,” Senator Schumer said in a statement. “Bernie Kerik is a tried-and-true New Yorker who understands our city, our state, our problems, and our needs. We look forward to working with him to bring greater help in terms of dollars and security for New York.”

“He’s a great choice,” Rep. Peter King, a Republican of Long Island, said in an interview. “Bernie has the experience as police commissioner of running a police organization. He also has the smarts of a street cop.”

Mr. King said Mr. Kerik will be aggressive enough to make sure the 2-year-old government department gets respect from more entrenched bureaucracies.

“You’ve still got to bang the CIA over the head. You’ve got to push the FBI. I don’t trust these people,” the congressman said. “He’s the type of guy who’s crashing through doors, locking people up as opposed to sitting behind a desk making policy.”

Another Republican congressman, Vito Fossella of Staten Island, also heralded the selection.

“This is great news for New York as we work in Congress to change the funding formula to get more homeland security money for the city,” Mr. Fossella said. “Bernie Kerik will fight terrorism with the same passion and commitment he displayed fighting crime in New York City.”

Mr. Kerik has close ties to another September 11 icon, Mayor Giuliani, who was once considered a possible contender for the homeland security job. The former mayor announced Wednesday that he plans to stay in the private sector for the next several years. When Mayor Bloomberg came into office in 2002, he asked Mr. Kerik to stay on, but the police commissioner chose instead to join Mr. Giuliani’s new business venture, Giuliani Partners.

Through a spokeswoman, Mr. Giuliani declined to comment yesterday on the imminent announcement. One of the former mayor’s advisers said, however, that Mr. Giuliani, who is seen as a possible candidate for the Republican presidential nomination in 2008, could benefit from having his longtime friend in Mr. Bush’s inner circle.

“This is a good thing for Rudy and a good thing for New York,” the adviser, who asked not to be named, said. “This is Giuliani’s reward for being a good soldier.”

The departing U.N. ambassador, Mr. Danforth, is a former senator from Missouri and an ordained minister. He accepted the job at Turtle Bay in the hope of advancing peace in troubled Sudan, where he had previously served as Mr. Bush’s point man. While the American diplomat managed to take the Security Council to Nairobi last week for a rare meeting in Africa, the crisis in Sudan has continued largely unabated.

Never quite at ease with U.N. diplomacy, the plainspoken Midwesterner often chafed at the ways of the staid institution. Last week, for example, Mr. Danforth reportedly balked at attending an annual U.N. event for “solidarity with the Palestinian people.” Only after he was pressured by fellow diplomats, who argued that his absence would be viewed as a breach of protocol, did he agree to take part.

Some speculated that Mr. Danforth’s resignation stemmed from his discomfort with the recent assaults in Washington on Secretary-General Annan. But the date of the ambassador’s resignation letter, November 22, was right after his return from Nairobi, and just before the oil-for-food scandal hit critical mass with The New York Sun’s disclosure about payments to the secretary-general’s son, Kojo Annan, and with Senator Coleman’s call for Mr. Annan to step aside.

The homeland security post would not be Mr. Kerik’s first high-profile assignment for the Bush White House. Last year, he spent six months in Iraq overseeing training for Iraqi police officers. At one point, Mr. Kerik even served as the country’s acting interior minister.

Mr. Kerik has also demonstrated his willingness to take up the political cudgel for Mr. Bush. The former police commissioner spoke at the Republican National Convention in New York in September. He also made campaign appearances on the president’s behalf and regularly joined in campaign conference calls aimed at rebutting Democratic attacks.

While Mr. Kerik may not be a household name in most of America, his personal story is so compelling that a movie studio, Miramax, is planning to produce a feature film about him.

Mr. Kerik grew up in a rundown area of Paterson, N.J. As a 4-year-old he was abandoned by his mother, who, he later discovered, was a prostitute. He never attended college but joined the Army, where he served as a military police officer in Korea.

After leaving the Army he worked for nearly four years as a security guard in Saudi Arabia, protecting the country’s royal family.

Upon returning to America, Mr. Kerik became a sheriff’s deputy in Pas saic County, N.J. In 1985 he was named warden of the Passaic jail. He left that job after a few months to join the NYPD.

As a narcotics detective, Mr. Kerik earned kudos for his undercover work busting drug dealers in Times Square, which was then much seedier than it is today.

He also helped infiltrate a Colombian drug gang as part of an investigation that led to the conviction of more than 60 members of the Cali cartel.

Mr. Kerik went on to serve at the city’s Corrections Department as deputy commissioner and commissioner. There, he received accolades for reducing violence at Rikers Island.

In August 2000, Mr. Giuliani appointed Mr. Kerik as police commissioner.

A spokesman for Miramax said plans for the film about Mr. Kerik were on track but might have to be revisited in light of his Cabinet nomination.

In another addition to his second term Cabinet, Mr. Bush announced yesterday that he will nominate the governor of Nebraska, Michael Johanns, as agriculture secretary.

“Governor Johanns is a man of action and of complete integrity. He knows how to bring people together to achieve results,” Mr. Bush said during a White House event yesterday.

If confirmed, Mr. Johanns, a second-term Republican who was raised at a dairy farm in Iowa, will replace the woman who served as agriculture chief throughout Mr. Bush’s first term, Ann Veneman. Ms. Veneman, who battled breast cancer during her tenure in the Bush administration, announced her resignation last month.