President Bush today named Bernard Kerik, a plain-spoken, street-wise policeman who led New York's police force during the Sept. 11 attacks, as secretary of the Department of Homeland Security.

Bush praised Kerik, who stood at his side in the White House Roosevelt Room, as "one of the most accomplished and effective leaders of law enforcement in America."

Kerik, 49, who spent 4 ½ months in Iraq last year seeking to rebuild the police force, would take over a department created less than two years ago to keep Americans safe from the threat of terrorist attacks. If confirmed by the Senate, he would replace Tom Ridge, the first head of the department, who announced his resignation Tuesday.

Citing Kerik's lengthy resume in law enforcement-in addition to serving as police commissioner in New York, he was a street cop, an undercover narcotics detective and corrections officer, among other police jobs-Bush said: "In every position, he has demonstrated a deep commitment to justice, a heart for the innocent and a record of great success."

"Every first responder defending our homeland will have a faithful ally in Bernie Kerik," Bush said.

In another Cabinet change, Bush on Thursday nominated Mike Johanns, a two-term Republican governor of Nebraska, as secretary of Agriculture. If confirmed by the Senate, Johanns, 54, would succeed Ann M. Veneman, a Californian.

Bush first met Johanns, the son of a dairy farmer, while governor of Texas. Johanns, a onetime Democrat and a former mayor of Lincoln, Neb., was elected to his current post in 1998 and reelected in 2002.

In addition, the U.S. ambassador to the United Nations, John C. Danforth, announced Thursday that he had submitted his resignation last month. A former Republican senator from Missouri, Danforth had served at the U.N. since the end of June.

In his Nov. 22 letter to Bush, Danforth, 68, cited his desire to return to private life but said he would remain available for short-term assignments. Before taking the U.N. post, he had served as Bush's special representative for peace in Sudan.

Since Bush's reelection a month ago, seven of his 15 Cabinet secretaries — of the departments of State, Justice, Energy, Commerce, Agriculture, Education and Homeland Security — have announced that they are leaving the administration. The White House has already named replacements for five of the positions.

More departures are expected in the coming days and weeks, administration officials said.

An acknowledged troublemaker in his youth — he dropped out of high school and got a young woman pregnant shortly after arriving in South Korea with the Army — Kerik has a reputation as a forceful leader who gets positive results.

If confirmed by the Senate, he would face the challenge of trying to defend against the possibility of another terrorist attack while seeking to finish the job of putting together the sprawling department, whose 180,000 employees were assembled from 22 agencies with vastly different missions.

Supporters of Kerik who watched him lead the New York Police Department through the attacks on the World Trade Center said he was up to the job.

"He has always been a very strong leader," said Patrick J. Lynch, president of the New York City Patrolmen's Benevolent Assn., the police union. "He understands security needs, especially in response to terrorism."

Sen. Charles E. Schumer (D-N.Y.) said in a statement that Kerik knew "the great needs and challenges this country faces in homeland security."

"He has a strong law enforcement background, and I believe will do an excellent job in fighting for the resources and focus that homeland security needs and deserves in our post-9/11 world," Schumer said.

Administration officials said Kerik's success in leading the police department during the city's most difficult weeks proved his qualifications to head Homeland Security.

"If you can run the New York City police department, you know how to do things," said one senior administration official, who did not want to be identified before an official announcement.