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The crux of the immigration deal gives illegal immigrants quick legal status but withholds a path to citizenship until certain criteria are met. But the bill also rewrites the legal immigration and guest-worker programs in language that was worked out between business groups and labor unions.

Sen. Charles E. Schumer, who is chief sponsor of the bill and one of the Gang of Eight who wrote the compromise, urged colleagues to stick with the deal.

“We have come to a careful balance on [high-tech visas],” the New York Democrat said. “What we tried to do in our proposal is to find a balance. As you’ve seen, some people think the balance is too far in the direction of labor. … Some people think the balance is too far in favor of high-tech.”

Senators also continued to struggle with the balance between legalizing illegal immigrants and strengthening security.

The entry-exit visa system was part of that fight.

Biometrics — immutable characteristics such as iris scans or fingerprints — have been a sticking point since 1996, when Congress passed legislation requiring a biometric identifier of every visitor as they enter and leave the U.S. That was supposed to be a way of making sure temporary visitors go home when their visas expire.

The 9/11 Commission renewed the push after the Sept. 11 attacks, when several of the hijackers were discovered to have overstayed their visas.

But current immigration bill requires photos rather than biometrics, and only at airports and seaports.

Sen. Jeff Sessions, Alabama Republican, offered an amendment to stick with the current biometric requirement, and to prevent newly legalized immigrants from getting citizenship until that is completed.

“This is a big hole in the system and it’s gone on for years and years,” Mr. Sessions said. “This is one reason the American people have so little confidence in any promises we make.”

But his proposal was defeated by a 12-6 vote, with 10 Democrats and two Republicans — both members of the Gang of Eight — in opposition.

“Current law is a concept. And there is apparently not a whole lot of will by Republicans or Democrats to make the concept a reality,” said Sen. Lindsey Graham of South Carolina, who was one of those Republicans.

Mr. Graham said he would like someday to get to biometric checks, but it’s proved to be too expensive. He said the bill they wrote is the best compromise between the demands of the law and the current practice.