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Other lessons Mr. Chishti said lawmakers have learned include requiring employers to use an electronic system to verify workers’ status, which should provide stronger enforcement checks, and reducing the cutoff date to less than two years for illegal immigrants to earn legal status.

Mr. Chishti said he would like to see less time because those who arrived after the Dec. 31, 2011, deadline again will germinate another illegal immigrant population.

The 1986 bill was sponsored in the House by Rep. Romano L. Mazzoli, Kentucky Democrat, and Sen. Alan K. Simpson, Wyoming Republican.

Mr. Simpson said the 1986 effort had a major setback when lawmakers on the right and left attacked their efforts to include a “secure identifier” to help employers verify workers’ status. One opponent, Rep. Edward R. Roybal, California Democrat, said it amounted to a national ID card and, according to Mr. Simpson, warned of a slippery slope to Nazi Germany-style tattoos.

The “Gang of Eight” that wrote this year’s bill is also wary of the charges of a national ID card, but Mr. Simpson said he believes they will have to include some sort of biometric identifier in the bill to make it work smoothly.

“What I hear them talking about are retina scans and fingerprints, and I ain’t seen one single article from either the right or left about slippery slope,” the retired senator said. “When the guy walks up to get a job and the employer says, ‘How do I know you’re you’ — at that point that has to be valid.”

Nine senators who were in the chamber in 1986 are still in office. Of those, six voted for the bill while two voted against it: Sens. Thad Cochran of Mississippi and Orrin G. Hatch of Utah. Another, Sen. Patrick J. Leahy, did not vote but, according to Congressional Quarterly Almanac, he was in favor of it.

Another 10 current senators were in the House in 1986 and voted on the bill, and they split with six opposing it and four voting in favor.

Since then, much has changed, including the minds of some key lawmakers. While Mr. Grassley is now an ardent supporter, Sen. John McCain, Arizona Republican, has gone from opponent to chief champion.

Perhaps most striking of all is Senate Majority Leader Harry Reid, Nevada Democrat, who voted against the law as a representative in 1986. A quarter-century later, Mr. Reid now says immigration is a deeply personal issue for him, and that’s why he has cleared the pathway for the bill, carving out the rest of the month of June for what he promised will be a freewheeling debate.

“I’ve devoted more time to immigration reform than any other issue over my career in Congress,” he said. “Each time I meet with constituents desperate for common-sense reform, my passion for fixing our broken immigration system is renewed.”