Representative F. James Sensenbrenner Jr. of Wisconsin, the Republican chairman of the House Judiciary Committee and a vocal opponent of the proposed legalization, also opposed the 1986 amnesty. Mr. Sensenbrenner and many House Republicans vehemently oppose this year's bill, leaving its future uncertain.

On Monday, Bill Frist, the Senate majority leader, moved to limit debate this week to ensure a final vote on the bill before Memorial Day. Meanwhile, lawmakers voted in favor of an amendment that would place National Guard troops on the United States border with Mexico.

"Since the '86 law did not succeed, people are understandably skeptical," said Mr. Specter, who is chairman of the Senate Judiciary Committee. "But this time, things are different."

In 1986, immigrant groups and many Democrats opposed the amnesty, fearing that restraints on employers would lead companies to avoid hiring legal immigrants or citizens with unusual names. Today, Democrats, immigrant groups and business leaders are among the strongest backers of the bill.

Supporters say better technology exists to create counterfeit-resistant cards to help employers distinguish between legal and illegal immigrants, secure the border and root out fraud. They say President Bush has demonstrated his commitment to enforcement by asking Congress for $1.9 billion to pay for putting up to 6,000 more National Guard troops on the Mexico border.

Structurally, though, there are still parallels to 1986. Then, as now, the legislation created two separate programs, a general legalization program and a program specifically for agricultural workers. And as in 1986, the agricultural program's rules in this year's bill are less stringent.

Under the legalization program, illegal immigrants would have to prove that they have lived in the United States for five years or more to qualify. Illegal immigrants who have been here two to five years could also apply, though they would have to depart the country first and participate in a temporary guest worker program before trying for legal residency. Both sets of applicants would have six months to apply.