His name has been mentioned as a possible 2008 presidential candidate.

Bush praised Ridge for his efforts to tighten security at the nation's airports, borders and ports and to safeguard critical infrastructure.

"In the fight against terrorism, he has played a vital role in protecting the American people from a real and ongoing threat," Bush said.

In addition to Romney and Leavitt, others being mentioned as possible candidates for the job include Asa Hutchinson, one of Ridge's undersecretaries; White House homeland security adviser Frances Townsend; former New Jersey Gov. Thomas Kean, who co-chaired the influential commission that investigated the Sept. 11 terrorist attacks; former New York City police commissioner Bernard Kerik; and Joe Allbaugh, former head of the Federal Emergency Management Agency.

Administration officials offered no hint of whom the president would nominate to replace Ridge. A Republican with close ties to the White House said the administration had a replacement in mind and was putting that person through the vetting process.

Many questions await Ridge's successor, including:

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• Whether to change the five-color alert system designed to help guide government response to terrorist threats.

  Critics contend that the system is too vague and geographically broad to be of much help to law enforcement or the public. A high "orange" alert last year, accompanied by a government recommendation that people buy duct tape for their windows, drew particular ridicule from comedians and cartoonists.

• Many of the nation's 429 commercial airports are considering a return to private security screeners because of management problems with the department's Transportation Security Administration. The TSA also had to scrap an ambitious passenger identification system because of privacy concerns and is working on a more modest replacement.

• Pressure is mounting for more aggressive policing of the U.S.-Mexico border, a difficult diplomatic challenge as President Bush is discussing a guest-worker program with Mexico.

• Coordinating the policies and personnel of the 22 agencies. On Monday, two government employee unions protested a department policy requiring workers to sign nondisclosure agreements for unclassified but sensitive information.

The department has also been dogged by charges of penny-pinching, failing to consolidate terrorist watch lists, and not doing enough to secure ports and railways.

But it has also received praise for restructuring immigration agencies, improving information sharing with state and local officials, and meeting difficult congressionally mandated deadlines.