Tom Ridge, the nation's first director of Homeland Security, announced Tuesday that he would resign his post effective Feb. 1, or earlier if a replacement is named.

Ridge, 59, a former Pennsylvania governor, said it was a "very difficult" decision. At a Washington news conference, he thanked President Bush for the "extraordinary opportunity to serve my country in this incredible period since Sept. 11, 2001."

"As I've said to you many times before, we have to be right a billion-plus times a year, meaning we have to make literally hundreds of thousands, if not millions, of decisions every year, or every day, and the terrorists only have to be right once," Ridge said in summarizing the task that he and his 180,000-strong umbrella agency were charged with.

Ridge will be the seventh person to leave Bush's Cabinet since the president's re-election earlier this month. Ridge was sworn in to the post in October 2001, weeks after the terrorist attacks that destroyed the World Trade Center in New York and damaged the Pentagon. Homeland Security became a Cabinet-level department in January 2003. His departure was widely expected and he had discussed leaving during his tenure. Ridge had been criticized at times for the government's color-coded terror alert system and was accused of politcizing the national security process to the benefit of Bush's re-election campaign.

Among the possible successors, according to

The New York Times

, are Asa Hutchinson, a former Arkansas congressman who once headed the Drug Enforcement Administration and now works as a deputy to Ridge; Frances Townsend, White House homeland security adviser; Mike Leavitt, the Environmental Protection Agency administrator and a former Utah governor with homeland security experience;

Massachusetts Gov. Mitt Romney, who is a member of the Homeland Security Advisory Committee; Bernard Kerik, New York City police commissioner during the 9/11 attacks who now advises the American presidential envoy in Iraq; and Joe Allbaugh, director of the Federal Emergency Management Agency.