Governor Spitzer’s decision to allow illegal immigrants in New York to obtain driver’s licenses is coming under increasing assault on several fronts.

Senate Republicans are accusing the governor of illegally implementing the policy, while at least one county clerk is vowing to ignore the changes and continue to deny licenses to applicants who cannot prove they are legal residents. Weighing in on the debate, Mayor Bloomberg yesterday expressed “concern” that the policy would jeopardize New York’s ability to comply with federal REAL ID regulations.

On Friday, Mr. Spitzer announced that the state, starting next year, would issue driver’s licenses to New Yorkers without regard to immigration status. New York’s more than 500,000 undocumented immigrants will be able to obtain a license by showing a current foreign passport verified by motor vehicle offices.

The policy, which reverses a Pataki-era requirement that New Yorkers lacking Social Security numbers submit federal documents proving they are ineligible for one, has sparked a heated debate over the merits of the new policy that has attracted national attention.

Opponents say awarding licenses to illegal immigrants would make it easier for terrorists to board airplanes, rent vans, open bank accounts, and enter government buildings, and would encourage identity theft. The Spitzer administration and immigrant groups say the policy would remove a hardship for a large, disadvantaged population and would strengthen security by bringing undocumented New Yorkers into a state system.

At least one county clerk is rebelling against the policy. The county clerk of Rensselaer, Frank Merola, said he is refusing to accept new state-issued equipment to scan foreign passports and vowed to reject applicants who fail to demonstrate their legal status.

“I’ve got 15 county employees, and they will not process that license,” he said. “I’m not going to do it. The governor’s going to really have to take a second look at this.”

Last week, Mr. Merola said he showed his license to a guard at the parking entrance of the Empire State Plaza concourse in Albany. “The guy behind me could be Osama bin Laden,” he said.

County clerks, he said, have scheduled an emergency meeting for October 4 to discuss how they plan to respond to the rules.

A Spitzer administration official said Mr. Merola and other clerks are agents of the Department of Motor Vehicles and as such must comply with the agency policy issued by Commissioner David Swarts.

Mr. Spitzer has been struggling to get his governing agenda back on course after dealing with several months of intense scrutiny over allegations that his administration improperly used state police to tarnish the Senate majority leader, Joseph Bruno.

The attack on the governor’s license policy comes after Mr. Spitzer was dealt a setback to his health care agenda when the Bush administration rejected his plan to extend public insurance to children in middle-income families. A slowdown in the economy has also put pressure on the governor to backtrack on promises to pour more money into education and a property tax cut program.

The challenge for Mr. Spitzer is to press ahead while facing a hostile Senate that is reluctant to hand the governor any policy victories. Republicans have responded with outrage to the new license rules and are vowing to overturn what they say is an illegal use of executive power.

“He’s doing exactly what he said he was going to do — ignore the Legislature,” a spokesman for Senate Republicans, John McArdle, said. “You can’t do this without the Legislature. It is arrogant. It is wrong and will not stand.”

Mr. McArdle pointed to a 12-year-old state traffic law that requires license applicants to provide Social Security numbers. Spitzer officials say the law, which was passed to track down people who owe child support, was never intended to deny licenses to undocumented immigrants.

In June, the Court of Appeals ruled that the Pataki administration did not exceed its statutory authority when in 2002 it required immigrants to provide federal documents proving legal status. Spitzer officials say the ruling permits them to roll back the governor’s policy.

Many Assembly Democrats, including Speaker Sheldon Silver, have come out in favor of the governor’s policy, making it unlikely that Republicans will be able to pass legislation overturning the governor’s directive.

Mr. Spitzer is not getting any backup from Mr. Bloomberg, who yesterday expressed alarm that the rules would interfere with New York’s compliance with federal REAL ID requirements that take effect in 2013.

“I am concerned about it,” Mr. Bloomberg told reporters. “This is the governor’s call.”

A Spitzer official said New York’s licenses did not comply with the new federal standards under the old rules and that New York may choose to opt out of the REAL ID program, which requires that states by 2013 replace their licenses with a standardized, tamper-proof ID. Regular licenses will no longer be accepted as identification at airports and federal facilities.