Key senators are exploring an immigration bill that would force every U.S. worker—citizen or not—to carry a high-tech identity card that could use fingerprints or other personal markers to prove a person's legal eligibility to work.

The idea, signaled only in vaguely worded language from senators crafting a bipartisan immigration bill, has privacy advocates and others concerned that the law would create a national identity card that, in time, could track Americans at airports, hospitals and through other facets of their lives.

The lawmakers haven't committed to the "biometric" ID card, and are wary of any element that might split the fragile coalition of Democrats, Republicans and outside organizations working toward agreement on a broad overhaul of immigration laws.

But at least five of the eight senators writing the bill have backed biometric ID cards in the past. At least three of them—Sens. Lindsey Graham (R., S.C.), John McCain (R., Ariz.) and Charles Schumer (D., N.Y.)—have said they support requiring the cards under the new law but are open to other options, aides say.

The goal is to ensure that employers can learn the legal status of all new hires quickly, part of the effort to dissuade illegal immigrants from entering the country or seeking jobs. A system now in place, called E-Verify, matches prospective hires to a database of Social Security numbers and other data, but it can be foiled at times when illegal immigrants give stolen names, dates of birth and Social Security numbers.