The economy of the country's largest city and the entire nation would collapse if illegal immigrants were deported en masse, New York Mayor Michael Bloomberg told a Senate committee hearing Wednesday.

New York City is home to more than 3 million immigrants, and a half-million of them came to this country illegally, Bloomberg testified.

"Although they broke the law by illegally crossing our borders ... our city's economy would be a shell of itself had they not, and it would collapse if they were deported," he said. "The same holds true for the nation."

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The hearing, led by Senate Judiciary Committee Chairman Arlen Specter, a Pennsylvania Republican, in Philadelphia, was one of several held nationwide as congressional Republicans take to the road to discuss overhauling immigration laws.

House Republican leaders called for the hearings last month in a blow to President George W. Bush's ambitious election-year plan that includes a guest worker program and path to citizenship for millions in this country illegally.

At a House subcommittee meeting at a Border Patrol station in San Diego, Republicans tried to focus on the potential for terrorists to enter the United States through Mexico.

"Immigration reform must be national security reform," said Rep. Ed Royce, the subcommittee chairman.

Democrats at the hearing sought to portray Republican immigration and border policies as failures.

"This is a charade," said Rep. Bob Filner, whose district includes the patrol station. "It's a cover-up for the fact that they can't produce a bill and they can't secure our borders."

Specter and fellow senators are trying to build support for a Senate bill that would allow a majority of the illegal immigrants to eventually become legal permanent residents and citizens after paying at least $3,250 (euro2,540) in fines, fees and back taxes and learning English.

The competing bill passed by the House focuses on enforcement and has no provisions for future guest workers or a citizenship path for illegal immigrants.

President George W. Bush, visiting a Dunkin' Donuts shop in Alexandria, Virginia, promoted his view that immigration legislation should provide some means for illegal immigrants who have been in the U.S. for a long time to stay.

"We cannot kick people out who have been here for awhile," Bush said, adding that they should be "treated with respect and dignity."

At the hearing in Philadelphia, a Pennsylvania mayor who has pushed for local ordinances cracking down on illegal immigrants asked for federal government assistance.

"We must dig deep into the city's accounts to pay for illegal immigrants, while illegal immigrants do not pay their fair share of taxes, either to the city, the county, the state or the country," said Louis J. Barletta, the mayor of Hazelton.

The panel that met in San Diego was set to meet again Friday in the border city of Laredo, Texas.