A federal judge yesterday barred the Bush administration from launching a crackdown Tuesday on U.S. employers who hire illegal immigrants while she considers a lawsuit by the AFL-CIO that charges that the plan will harm citizens and other legal workers.

The ruling, issued by U.S. District Judge Maxine M. Chesney in San Francisco, prohibits the Department of Homeland Security from starting to mail notices to 140,000 employers about suspect Social Security numbers. The "no- match" letters warn of penalties employers face by having discrepancies in their paperwork.

The order was a victory for the labor federation and the American Civil Liberties Union, which filed suit alleging that DHS is overstepping its authority to enforce immigration laws and is misusing a Social Security Administration database. They allege that the letters are an effort to pressure businesses to fire workers whose documents are flagged and could expose countless immigrant workers -- including law-abiding citizens and legal residents -- to job discrimination.

Chesney granted the request for a temporary restraining order against the government, saying the court needs "breathing room" before issuing a decision on the DHS plan. She set a hearing for Oct. 1.

The ruling dealt at least a temporary political setback to President Bush, who announced the workplace initiative Aug. 9 as the centerpiece of a renewed enforcement push. The White House acted after its drive for a broad immigration overhaul failed in Congress this summer.

"We're disappointed by the delay and expect to prevail once the court has the benefit of full briefing and argument," DHS spokeswoman Laura Keehner said, adding that the department will continue other enforcement actions.

There are errors in the Social Security records of an estimated 12.7 million native-born U.S. citizens, 250,000 foreign-born citizens and 4.8 million legal immigrants, potentially creating a paralyzing bureaucratic nightmare for workers and Social Security offices if their records are dragged into the sweep, union representatives said.

In recent years, the Social Security Administration has found that up to 10 percent of workers have suspect numbers, whether because of fraud, innocent typographical errors, confusion over name changes, multiple surnames or other reasons. Under the DHS proposal, starting Sept. 14 employers would have 90 days to resolve questions concerning such employees' identities or to fire them. If they do not comply, businesses face the possibility of fines and even criminal penalties for knowingly violating federal law that bars employing illegal workers.

Leading U.S. business groups say the letters will trigger extensive workplace disruptions and plant and company closings this winter across the agriculture, meatpacking, construction, lodging, hospitality and service sectors. They have asked the Bush administration for a 180-day delay and for answers to 80 questions about how businesses can comply with immigration laws as well as anti-discrimination statutes.