WASHINGTON - In a surprise move, President Bush on Tuesday nominated Michael Chertoff, a federal appeals court judge and former high-ranking Justice Department official, to be homeland security secretary.

Chertoff has a reputation as an aggressive federal prosecutor, going after mobsters, a president and terrorists.

In tapping Chertoff, Bush selected a highly respected legal mind who has little experience in managing something as large and unwieldy as the Department of Homeland Security.

The two-year-old department with 22 agencies and 180,000 employees has been described as a dysfunctional bureaucracy.

However Chertoff brings legal expertise in dealing with terrorism.

Chertoff ran the Justice Department's criminal division from 2001 to 2003 and helped craft the administration's legal strategy after the Sept. 11, 2001, terrorist attacks.

''On Sept. 11, 2001, I joined members of dozens of federal agencies in responding to the deadliest single attack on American civilians ever,'' Chertoff said Tuesday.

''If confirmed as secretary, I'll be proud to stand again with the men and women who form our front line against terror,'' he said.

Chertoff is Bush's second pick to replace outgoing Homeland Security Secretary Tom Ridge.

His first choice, former New York City Police Commissioner Bernard Kerik, withdrew from consideration after embarrassing the White House with an admission that he had not paid taxes for a nanny who may have been an illegal immigrant.

The Kerik episode prompted questions about how thorough the administration was vetting its nominees.

Bush on Tuesday stressed that there should no problems or surprises with Chertoff's confirmation.

''He's been confirmed by the Senate three times,'' Bush said. The president praised Chertoff's qualifications.

''He's faced countless challenging decisions and has helped to protect his fellow Americans while protecting their civil liberties,'' Bush said as he announced Chertoff's selection to reporters in the White House's Roosevelt Room.

Chertoff's selection caught several lawmakers and homeland security experts off guard.

Among those previously mentioned as likely candidates were Asa Hutchinson, the Homeland Security department's undersecretary for border and transportation security, and Fran Townsend, the White House's homeland security advisor.

''I hadn't heard his [Chertoff's] name mentioned,'' said Rep. Peter King, R-N.Y., who had supported Kerik.

``But it's one of those deals that after the fact it makes sense. He has experience dealing with Congress, he's got law enforcement experience and he has experience putting bad guys in jail.''

Sen. Jon Corzine, D-N.J., called Chertoff ``one of the most able people and public servants I have ever known.''

If confirmed, Chertoff will face a daunting task.

Former Homeland Security Inspector General Clark Kent Ervin said in published reports that, despite spending millions of dollars, the nation's ports and airports remain vulnerable to terrorists.

Chertoff ''has strong bipartisan support, great intelligence, integrity and energy -- and that will help make up for shortcomings in expertise,'' said Michael Greenberger, director of the University of Maryland's Center for Health and Homeland Security.

``He's a person who should be able to attract high-caliber people to a department that has an alarming number of high-level vacancies.''

Chertoff was appointed by Bush in 2003 to the Third U.S. Circuit Court of Appeals -- which serves

Delaware, Pennsylvania, New Jersey and the U.S. Virgin Islands.

His reputation as an aggressive prosecutor was earned on several fronts.

He pursued mob bosses in New York and New Jersey in the 1980s.

Later he was Republican counsel during the Senate Whitewater investigation of former President Bill Clinton.

And after the Sept. 11 attacks he helped devise legal strategies used by the Justice Department.