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Homeland Security chief Tom Ridge (search) announced his resignation Tuesday in a surprise development that ends his 23-month stint as the first secretary of the terror-fighting agency.

"I will always be grateful for the opportunity to serve my country with the president as its leader," Ridge told reporters in an afternoon press conference. He said he will stay on the job until Feb. 1 or until his successor is confirmed, whichever comes first.

"Homeland Security has never been to me just a department — it's about the integration of a country, taking the resources, the capabilities and the capacities we have ... and making sure they are all engaged in a fundamental way, in a certain way that collectively we as a country are safer and more secure," he said.

Ridge handed in his formal resignation to President Bush on Tuesday after meeting with the president and chief of staff Andy Card before the president left for Canada on a state visit. Ridge also had what has been described as an emotional meeting to inform senior leadership of his plans to resign.

In an e-mail circulated to senior Homeland Security Department (search) officials before his announcement, Ridge praised the department as "an extraordinary organization that each day contributes to keeping America safe and free." He also said he was privileged to work with the department's 180,000 employees "who go to work every day dedicated to making our country better and more secure."

"We thank him for his service, it has been a mammoth undertaking," said Rep. Christopher Shays, R-Conn., upon learning of Ridge's decision.

In October 2001, Ridge became the nation's first White House homeland security adviser, leading a massive undertaking to rethink all aspects of security within the U.S. borders in the wake of the terror attacks of September 2001.

Congress subsequently passed legislation establishing the Homeland Security Department, merging 180,000 employees from 22 government agencies. Ridge became the department's first secretary in January 2003.

He has presided over six national "orange alerts" when the government boosted security out of concern that an attack may be coming. An attack in the United States never happened on his watch.

Ridge has said, however, that he believes an assault by the Al Qaeda (search) terrorist network was averted last summer during the July 4th holiday period, when intelligence reports indicated terrorists might be targeting international flights to the United States. Passenger manifests were scrutinized and flights were canceled.

Yet Ridge fought criticism leading up to the election from those who said he was using terror warnings to boost support for Bush. Ridge repeatedly said: "We don't do politics in the Department of Homeland Security."

Ridge, 59, has privately expressed his interest in moving out of the time-consuming, stressful post. He said he wants to spend more time with his family, participating in activities that his busy schedule has prevented him from pursuing.

Those who know Ridge add that his loyalty to Bush was always a factor to consider when deciding to resign. The two became friends when they both served as governors.

"This is not much of a surprise. We've known for some time that Secretary Ridge has not been crazy about living here in Washington and it's such a demanding job, so much pressure, really walking on eggshells all the time," said Washington Post reporter and FOX News analyst Ceci Connelly.

Ridge, who was raised in Erie, Pa., received a degree from Harvard before being drafted into the Vietnam War. After the war, he earned a law degree and entered private practice in Pennsylvania. He later served as an assistant district attorney and ran for Congress in 1982.

Ridge was re-elected to Congress five times. He became the Pennsylvania governor in 1995, leaving the state capital in October 2001 after the White House called him to become a top adviser in the aftermath of the Sept. 11 attacks.

Possible replacements for Ridge include Bernard Kerik (search), interim minister of the interior for Iraq and former New York City police commissioner; Federal Emergency Management Agency Director Joe Allbaugh (search); Environmental Protection Agency Administrator Mike Leavitt (search) and White House homeland security adviser Fran Townsend. Asa Hutchinson, undersecretary for border and transportation security in the Homeland Security Department, is also said to be interested in the job.

Ridge is the seventh Cabinet secretary to resign. Also departing are Attorney General John Ashcroft, Commerce Secretary Don Evans, Education Secretary Rod Paige, Agriculture Secretary Ann Veneman, Secretary of State Colin Powell and Energy Secretary Spencer Abraham. Bush has already named four nominees to fill those spots.

Connelly said that it may be difficult to find a replacement for Ridge.

"It's not a very appealing assignment quite frankly, because if there is a terrorist attack, which many have expected for a long time, certainly the person in that position takes some blame, rightly or wrongly," she said.

Sen. Charles Schumer, D-N.Y., agreed that the position is not attractive, in part because homeland security faces many challenges and a tight $33 billion budget.

"Tom Ridge is a decent man and a fine public servant but unfortunately was not given the leeway or resources to tighten up homeland security the way it should be done. We hope that whoever the administration chooses to succeed him will be given the tools needed to really do the job," Schumer said in a written statement.

The Associated Press contributed to this report.