The Senate voted Wednesday to slash the number of foreign workers who could come to the U.S. on temporary visas as part of a broad bipartisan immigration bill.

A new guest worker program would be capped at 200,000 a year under the proposal, which passed 74-24 over strong opposition by the Bush administration.

Commerce Secretary Carlos Gutierrez said the change, proposed by Sen. Jeff Bingaman, D-N.M., would interfere with a "central component" of the White House-backed immigration measure. That plan provided for 400,000 worker visas annually, plus an option to increase that number to 600,000 if market conditions demand it.

"The Bingaman amendment would eliminate this critical flexibility and cut the size of the temporary worker program in half," Gutierrez said in a statement.

His comments came as the administration urged the Senate to approve the immigration legislation despite fresh criticism from presidential hopefuls and lawmakers in both parties.

"The proposal offers a much-needed solution for our nation's broken immigration system," the White House budget office said in a statement. "This proposal would deliver an immigration system that is secure, productive, orderly and fair."

The measure would grant an estimated 12 million unlawful immigrants quick legal status, toughen border security. It also would create a new workplace verification system to bar undocumented workers from getting jobs.

It would set up a point system for future immigration applicants that would place less emphasis on family connections and more on education and skills in demand by U.S. businesses.

Sen. Barack Obama of Illinois, a Democratic presidential hopeful, announced plans to challenge the point system, saying it devalued family.

The scheme "constitutes at minimum a radical experiment in social engineering and a departure from our tradition of having family and employers invite immigrants to come," Obama said, adding that he would work to phase it out.

A 2008 rival, New York Sen. Hillary Rodham Clinton, said she would seek to lift new caps the measure would place on visas for family members of legal permanent residents.

Republicans sought to respond to conservative critics by trying to bolster security provisions and make it more difficult for illegal immigrants to get on a path to citizenship.

Sen. Lindsey Graham proposed cracking down on illegal border crossers with mandatory prison sentences.

"Everyone needs to know that America is changing its immigration laws. We're going to be serious about enforcing them. If you break our laws, you do so at your own peril, and you will lose your freedom," said Graham, R-S.C.

The Senate was also considering a proposal by Sen. Charles E. Grassley, R-Iowa, that would allow visas to be revoked without court review.

"Current law allows aliens to run to the steps of our country's courthouses and take advantage of our system," Grassley said. He said potential terrorists could stay in the country unless lawmakers approved his change.

Democrats heard objections from labor unions and immigrant groups about the guest worker program and focused on shrinking or altering it.

The temporary worker plan would bring in laborers to stay for up to three two-year stints, provided they left the United States for a year between each stay. A Democratic attempt to strip the program altogether failed Tuesday in the first major test of the fragile immigration compromise.

The conservatives, liberals and centrists who worked out the deal are struggling to keep it intact.

Senate leaders in both parties, however, say it is important to have a vigorous debate. They have postponed a final vote until June.