WASHINGTON - Opponents of immigration reform legislation in the Senate are warning that it could provide an invitation to fraud that would draw an influx of illegal border crossers in search of a lifetime pass to settle in the United States.

Backers of the measure, already battling charges from conservatives that it amounts to amnesty, acknowledge the problem but promise to fix it before the immigration package becomes law. The problem is that the legislation, which would provide legal status to millions of persons who entered the country illegally before last Jan. 1, sets 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 'vamanos' - let's go," immigration scholar George Grayson of William & Mary University, warned.

"It sounds like an open invitation to fraud," said Sen. John Cornyn, a Texas Republican, who has been broadly critical of the legislation.

It was forged in a hard-fought compromise between Democrats, led by liberal Sen. Edward Kennedy of Massachusetts, and Republicans, led by conservative Sen. Jon Kyl of Arizona, and joined by the White House.

The budding controversy reflects the inherent difficulty in establishing standards of proof for millions of persons who have been living in the shadows, many of them with false identities in an all cash economy.

But it has been largely ignored as public attention has focused primarily on what critics call an amnesty. Advocates say it 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 reform legislation is intended to bring the estimated 12 million illegal immigrants out of the shadows. But Kyl acknowledged he is concerned that number could grow because of fraud.

Asked how he thought the problem could be addressed, he said, "I'm not sure." But he said it would be taken up along with other flaws in the bill when the Senate returns following next week's recess.

California Democratic Sen. Dianne Feinstein also played a central role in forging the proposal, which was debated this week and will be taken up by the Senate following next week's recess.

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."

She said, "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."

Stewart Baker, assistant secretary for policy at the Department of Homeland Security, which will administer the program, agreed, saying, "That is the line we have to draw." He added, "I'm sure there will be mistakes on both sides of the line."

Meanwhile, Baker said DHS is aware that some persons may be drawn into the country illegally by the hope of buying phony documents that will open the door to permanent residence 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 fed into a data base where applicants for legalization will be checked. The application would be denied for anyone detained after Jan. 1, he said.

Concerns that the controversy is vulnerable to fraud are highlighted against the historical background of rampant fraud that followed passage of the 1986 amnesty legislation.

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

That SAW program was the target of massive abuse, said immigration scholar Philip Martin of the University of California at Davis.

While Congress had expected about 375,000 persons would apply under the program, nearly three times that many applied and 1.2 million were granted amnesty. Many, said Martin, bought notarized letters from persons who claimed to have employed them for the requisite period.

"A typical letter said the person had picked tomatoes for 92 days in Stockton," said Martin. 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 today's document counterfeiters have computer-based tools far more sophisticated than what was available at the time of the 1986 amnesty.

"Two decades ago was the Orville and Wilbur Wright era of falsifying documents," he said, referring to the two pioneers of aviation. "Now we're into the Stealth Bomber period."