Day in and day out, as the immigration debate boils, the halls of Congress are haunted by the specter of Senate Bill 1200, the failed amnesty legislation of 1986.

President Ronald Reagan signed that bill into law with great fanfare amid promises that it would grant legal status to illegal immigrants, crack down on employers who hired illegal workers and secure the border once and for all. Instead, fraudulent applications tainted the process, many employers continued their illicit hiring practices, and illegal immigration surged.

Today, senators who hope to put the nation's illegal immigrants on a path to citizenship say they have learned from the past. But some members of Congress and former immigration officials fear history will repeat itself.

Even some who favor legalization warn that the current bill, which requires illegal immigrants to submit affidavits, rent receipts and other documents as proof of eligibility, may fuel a wave of fraudulent documents and applications.

Demetrios G. Papademetriou, who studied the 1986 amnesty at the Labor Department in the first Bush administration, said he was encouraged when he heard that the Senate was close to granting legal status to illegal workers. But Dr. Papademetriou, who is now president of the Migration Policy Institute, a nonpartisan research group in Washington, said his heart sank when he learned about the legalization process, which he believes will create a market for counterfeit documents.

In the late 1980's, immigration officials approved more than 90 percent of the 1.3 million amnesty applications for a specialized program for agricultural workers, even though they had identified possible fraud in nearly a third of those applications. The general amnesty, which legalized 1.7 million people, worked much more efficiently, though some of its applications raised similar concerns.

Dr. Papademetriou, recalling the difficulties 20 years ago, said: "We're going back to 1986. Do we ever learn anything?"