Updated

Now that Tom Ridge has formally announced he will leave his post as the nation's homeland security chief, the battle is on to guess his successor.

President Bush will need "a great leader, someone who has experienced not only academia or the private sector but has experienced government," Rep. Pete Sessions, R-Texas, told FOX News.

"What we need to do is focus on a person who really can bring together the avenues of opportunity. This is a tough job, and it's going to require a person who can make decisions, who can work with the Congress, but most of all communicate with the American people about the threat that this nation still faces on a day-to-day basis. I think the president is going to make a wise choice," added Sessions, who sits on the House Select Committee on Homeland Security.

Ridge announced his resignation Tuesday and acknowledged the frustrations of working out the kinks in the broadest government reorganization in half a century.

"I like going to work every day," Ridge said, before adding, "There are certain days I've enjoyed it even more."

Ridge said he will remain in the job until Feb. 1 or until the Senate confirms his successor, whichever comes first. Among those cited as potential candidates are Bernard Kerik (search), the former New York City police commissioner who helped rebuild Iraq's police force; former Federal Emergency Management Agency Director Joe Allbaugh (search); Environmental Protection Agency Administrator Mike Leavitt (search); New York Police Commissioner Ray Kelly (search); and White House homeland security adviser Fran Townsend (search).

Others are also believed to be interested in the job, including Asa Hutchinson (search), undersecretary for border and transportation security in the Homeland Security Department.

Sessions opined that Allbaugh would be a good choice, as would Hutchinson or Kelly, especially since the latter two are likely to follow the letter of the law.

"They want to make sure that they take on not just the avenues of intelligence but also the American people and ask the American people to work with them," the congressman said. "I think both of these gentlemen once again would be very fine choices."

Sen. Richard Shelby, R-Ala., told FOX News that Hutchinson may be a good fit since he's worked so closely with Ridge. But he said Kelly, Kerik or Giuliani also would be "excellent" choices.

Critics said Ridge's legacy is mixed at DHS, a collection of 22 disparate federal agencies with more than 180,000 employees. It includes Citizenship and Immigration Services, the Immigration and Customs Enforcement bureau, the Coast Guard, the Secret Service and the Transportation Security Administration, with a combined budget of $33 billion.

Some say Ridge, the former governor of Pennsylvania, took on a no-win situation and often did not have enough resources.

"He was dealt an impossible hand," said Richard Clarke, the former top counterterrorism adviser to Bush and former President Clinton who resigned last year. "He was not allowed to make some of the key decisions about the beginning of the department. I think anyone would have failed under those circumstances."

But despite the circumstances, many pointed to Ridge as the reason why the United States has had so many successes in homeland security since the Sept. 11 attacks.

"Tom Ridge has put a lot of agencies together, I think they're working better than they were but this is a work in progress," Shelby said. "Perhaps you don't have enough resources but you'll never have enough resources to create bunkers for everyone in this country — it's something we don't want."

DHS Chief Needs to 'Knock Heads' Together

Although the Homeland Security Department originally was envisioned as a broker of intelligence after the 2001 terror attacks, the Bush administration opted to establish a new terrorist screening center and terrorist threat integration center, set up at the FBI and CIA respectively.

"Ridge is a personality. He engendered extreme loyalty in headquarters staff," said Dan Prieto, a research director at Harvard University and former Democratic staff member for the House Homeland Security Committee. "Was he the right person to go out on a limb, knock heads and make sure the department achieved this goal? I would argue that he wasn't."

Even loyalists to Ridge acknowledge the next secretary must work to unify the organization, which still uses hundreds of different computer networks for systems such as intelligence, accounting, procurement and hiring. Auditors have complained about the department's spending controls and the security of its own computer networks. It can take months at DHS to hire key employees for important positions.

"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," Sen. Charles Schumer, D-N.Y., said in a statement after Ridge's resignation. "We hope that whoever the administration chooses to succeed him will be given the tools needed to really do the job."

Ridge: We're Safer Today

Ridge will be remembered for his terror alerts and tutorials about how to prepare for possible attacks, including the controversial "disaster kits" that caused last year's run on duct tape and plastic sheeting. The color-coded national terror alert system has come under a firestorm of criticism since it was implemented, with questions being asked about whether the system is not specific enough.

"I was alerted by the upgrading of the colors into various modes, but at the same time, when nothing happens, people said, 'Well, gosh, they're crying wolf, wolf, wolf, and the wolf doesn't come,' and If you didn't warn, you'd be damned, and you'd be damned if you don’t," Shelby said.

Ridge said he was certain the country is safer today than before the suicide hijackings on Sept. 11, 2001, killed nearly 3,000 people in New York, Washington and Pennsylvania.

"I am confident that the terrorists are aware that from the curb to the cockpit we've got additional security measures that didn't exist a couple years ago," Ridge told reporters Tuesday at the department's Washington office.

Bush said Ridge's efforts resulted in "safer skies, increased border and port security and enhanced measures to safeguard our critical infrastructure and the American public."

Ridge sent his letter of resignation to Bush at midday Tuesday, after attending a morning White House threat briefing with CIA and FBI officials. He thanked Bush for giving him the opportunity to fight back against terrorists. He recalled that the passengers on Flight 93, who forced their hijacked plane down in a Pennsylvania field, had also fought back.

"There will always be more to do, but today, America is significantly stronger and safer than ever before," Ridge wrote Bush.

Ridge is the seventh of Bush's 15-member Cabinet to announce they won't be part of the second term.

FOX News' Liza Porteus and The Associated Press contributed to this report.