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“Administrator O’Keefe has previously indicated to us that he is planning on leaving,” said White House spokesman Scott McClellan. “The president believes he has done a great job over at NASA.”

According to sources familiar with discussions for his replacement, Air Force Lt. Gen. Ronald Kadish, who directed the effort to develop a system to shield the country from a missile attack, is at the top of the list. Others include former shuttle astronauts Ron Sega, Charles Bolden and Robert Crippen.

Although Mr. O’Keefe has embraced a new space effort, envisioned by Mr. Bush, that would send manned missions to the moon and Mars, the space program has been eclipsed by other pressing needs, including the war in Iraq and tax cuts.

The administration is moving more slowly to replace Mr. Kerik as the nominee to head the Homeland Security Department after Tom Ridge resigned. Among candidates were Asa Hutchison, the undersecretary of homeland security; Frances Fragos Townsend, the White House homeland-security adviser; and the former chief of the Federal Emergency Management Agency, Joe M. Allbaugh. Also reportedly in the running is Sen. Joe Lieberman, Connecticut Democrat.

Mr. Kerik abruptly withdrew Friday night, citing immigration problems with a family housekeeper. But the New York Daily News reported yesterday that Mr. Kerik had simultaneously conducted two extramarital affairs. There also were reports about connections he had to a construction company with reputed ties to organized crime.

A candidate for replacing Mr. Leavitt as EPA administrator was the chairman of the White House Council on Environmental Quality, James Connaughton.

Cheryl Wetzstein and Amy Fagan contributed to this report.