WASHINGTON, Nov. 25 — President Bush signed legislation today creating a Department of Homeland Security and named Tom Ridge, the former Pennsylvania governor who has been the White House's domestic security coordinator, to run it.

Carol Powers for The New York Times Calling him "the right man for this new and great responsibility," President Bush nominated Tom Ridge to head the Department of Homeland Security.

Mr. Bush's signature on the bill, which won final Congressional approval last week after a bitter political fight, set in motion a vast bureaucratic reorganization that the president said would "focus the full resources of the American government on the safety of the American people."

By nightfall, 60 days before he was required by law to do so, Mr. Bush had sent to Congress his detailed plan for bringing the department into being. The plan called for getting the department largely up and running by March 1, with the process to be completed, at least in organizational terms, by Sept. 30.

In announcing that he would nominate Mr. Ridge as the first secretary of homeland security, Mr. Bush was entrusting him not just to oversee what is sure to be a difficult merger of disparate government agencies but also to elevate defense of Americans at home to a new level in the face of what officials say are ever-evolving terrorist threats.

"He has a monumental task in front of him," said Senator Joseph I. Lieberman, the Connecticut Democrat who was one of the original proponents of creating the cabinet-level department. "It's like asking Noah to build the ark after the rain has started to fall."

Mr. Bush also began filling other top posts in the department. He nominated Gordon R. England, a former military contracting executive who is the Navy secretary, to be Mr. Ridge's deputy, and Asa Hutchinson, a former Republican congressman from Arkansas who is the administrator of the Drug Enforcement Administration, as undersecretary for border and transportation security.

The birth of the department flowed from a bipartisan consensus after last year's terrorist attacks that the nation needed to do more to protect its citizens at home. It will require the largest reshuffling of governmental responsibilities since the founding of the Defense Department after World War II, a process sure to encompass turf battles and culture clashes even as the country parries a steady stream of terrorist threats and girds for possible war with Iraq.

It will bring together nearly 170,000 workers from 22 agencies with widely varying histories and missions, like the Coast Guard, the Secret Service, the federal security guards in airports and the Customs Service. The goal is to improve security along and within the nation's borders, strengthen the ability of federal, state and local authorities to respond to an attack, better focus research into nuclear, chemical and biological threats and more rigorously assess intelligence about terrorists.

"The continuing threat of terrorism, the threat of mass murder on our own soil, will be met with a unified, effective response," said Mr. Bush, who had at first resisted calls for creation of the new cabinet-level department and embraced them only after political pressure mounted last spring.

"Dozens of agencies charged with homeland security will now be located within one cabinet department with the mandate and legal authority to protect our people," he said as he prepared to sign the bill in the East Room before an audience of cabinet members, Republicans and Democrats from Capitol Hill and law enforcement officials from around the country. "America will be better able to respond to any future attacks, to reduce our vulnerability and, most important, prevent the terrorists from taking innocent American lives."

But Mr. Bush also injected a note of caution, saying, "No department of government can completely guarantee our safety against ruthless killers, who move and plot in shadows."

Led by Clay Johnson, the White House personnel director, a transition team has been working since summer to allow the administration to move quickly to deal with details like establishing systems for better evaluating intelligence reports.

To help oversee the integration into the new department of one of its most troubled components, the Immigration and Naturalization Service, Mr. Bush today named Michael Garcia, an assistant commerce secretary and former federal prosecutor, as acting immigration commissioner. The service has come under widespread criticism for failing to keep track of immigrants, most recently in a report last week by the General Accounting Office that said immigration authorities had been unable to find nearly half of more than 4,000 registered immigrants the government wanted to interview after the Sept. 11 terrorist attacks.

With many agencies struggling to deal with deeply entrenched problems, experts said improvements to domestic security would not take place with the stroke of Mr. Bush's pen today or even the submission of the reorganization plan. They said it could be years before the department could be expected to operate at full effectiveness.

"The first challenge is to lower expectations," said Paul C. Light, who studies government organization at the Brookings Institution, a liberal-leaning research organization. "People should think they will be safer, but remember we have a long way to go."

Some Democrats also suggested that there were risks that the reorganization could be a distraction to some of the agencies most directly involved in domestic security, especially since government employee unions are already concerned that Mr. Bush wants to scale back Civil Service protections for workers in the new department.

Moreover, some Democrats said, the administration is not providing the department with the money it will need to do its job effectively.

"We didn't reorganize the Pentagon in the middle of World War II," said Representative David R. Obey of Wisconsin, the senior Democrat on the House Appropriations Committee. He said the administration was "shortchanging" the department in areas like safeguarding nuclear materials and assisting state and local governments with the financial burden of the roles they must take on.

The bill came to Mr. Bush's desk only after generating deep partisan differences through much of the year. The idea of a cabinet-level department was first pushed more than a year ago by Mr. Lieberman and other Democrats on the Senate Governmental Affairs Committee. Mr. Bush said at the time that the job could be better done, at least initially, by an office within the White House rather than by creating a new bureaucracy.

Mr. Bush endorsed the idea in June, after pressure grew in both parties to address weaknesses in the government's performance in battling terrorism. But the two parties in Congress then became enmeshed in an argument over whether to grant the president broad powers to hire and fire federal workers and move them among jobs.

Republicans said Democrats were obstructing the bill at the bidding of union leaders representing government employees, an assertion that Republicans used to attack two incumbent Democratic senators in the election this fall, Max Cleland of Georgia and Jean Carnahan of Missouri. Both lost, and Democrats largely gave in to Mr. Bush's demands after the election, leaving them with some bitterness about what they viewed as politicization of the issue by Mr. Bush and his party.

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