Backers of the measure, already battling charges from conservatives who say the proposal amounts to an amnesty, acknowledge the problem and promise to fix it before the bill becomes law.

The problem is that the legislation, which would provide legal status to millions of people who entered the country illegally before Jan. 1, 2007, would establish loose standards for proving length of stay. Unless the standards are toughened, critics contend, there may be a rush to the border and a new flood of fraudulent documents.

“All across Mexico and other countries, the word will be vámonos – let's go,” said George Grayson, an immigration expert at the College of William & Mary.

Sen. John Cornyn, a Texas Republican, has been broadly critical of the legislation, which was forged in a hard-fought compromise between Democrats led by Sen. Edward Kennedy of Massachusetts and Republicans led by Sen. Jon Kyl of Arizona and joined by the White House.

“It sounds like an open invitation to fraud,” Cornyn said.

The budding controversy reflects the inherent difficulty in establishing standards of proof for millions of people who have been living with false identities in an all-cash economy.

The issue has been largely ignored as public attention has focused primarily on what critics call an amnesty and advocates say is the only practical way of solving the problems associated with a huge and steadily expanding population of illegal immigrants drawn primarily by low-wage jobs in service industries.

The immigration overhaul is intended to bring the estimated 12 million illegal immigrants out of the shadows. Kyl acknowledged he is concerned that number could grow because of fraud.

Kyl said recently that he wasn't sure how the problem could be addressed, but it would be taken up along with other flaws in the bill when the Senate returns after this week's recess.

California Democrat Sen. Dianne Feinstein also played a central role in forging the proposal. In a written response to questions from Copley News Service, Feinstein defended the bill's language on documents as “part of the carefully negotiated compromise.”

“It may not be perfect, but it reflects the balance that must be struck between enforcement and the need to bring workers out of the shadows,” she wrote.

Stewart Baker, assistant secretary for policy at the Department of Homeland Security, which will administer the program, agreed.

“That is the line we have to draw,” Baker said. “I'm sure there will be mistakes on both sides of the line.”

Baker said Homeland Security is aware that some people may be drawn into the country illegally by the hope of buying phony documents that will open the door to permanent residency in the United States.

“Obviously, we have to worry about the fact that people will be attracted by the opportunity to commit fraud,” he said.

Baker said the ongoing buildup of the Border Patrol has raised the probability that illegal crossers will be detained. All detainees are fingerprinted, he said, and that information is fed into a database against which applicants for legalization will be checked.

Concerns about vulnerability to fraud are highlighted against the background of problems that followed passage of immigration legislation in 1986.

In addition to providing general amnesty to people who had been in the country for five years, that legislation created a Special Agricultural Worker amnesty for those who had worked 90 days in the fields during the previous year.

The program soon became a target for widespread fraud.

The most notorious instance involved Mahmud Abouhalima, an Egyptian who entered the country on a tourist visa in 1985 and worked illegally as a New York taxi driver after his tourist visa expired. Abouhalima's application for amnesty as an agricultural worker was approved and he later was able to travel to Afghanistan, where he received terrorist training. He later was convicted for his role in planning the 1993 bombing of the World Trade Center.

Congress had expected about 375,000 people would seek amnesty under the agricultural program, said Philip Martin, an economist and immigration expert at the University of California Davis. That estimate was far too low; nearly 1.3 million people were granted amnesty. Many, Martin said, bought notarized letters from people who claimed to have employed them for the requisite period.

“A typical letter said the person had picked tomatoes for 92 days in Stockton,” Martin said.

Such letters were often accepted even though the tomato season doesn't last that long, he said.

The current Senate legislation allows applicants for a Z visa, which would put holders on a path to citizenship, to prove their length of stay with a variety of documents. Among the acceptable documents are affidavits signed by a non-relative. Others include records from employers, unions or day-labor centers.

Grayson said he expected the program's administrators to be flooded by newly arrived illegal immigrants armed with phony documents. He noted that document counterfeiters have far more sophisticated tools now than what was available in 1986.

“Two decades ago was the Orville and Wilbur Wright era of falsifying documents,” he said, referring to the aviation pioneers. “Now we're into the stealth bomber period.”