A bipartisan Senate group has agreed on a sweeping legislative proposal that would represent the most ambitious overhaul of the U.S. immigration system in three decades. The Washington Post will be examining portions of the bill on Post Politics in a series of blog entries.

One area of the immigration bill that will be proposed this week by a bipartisan group of eight senators that is already drawing fire from civil libertarians is the legislation's dramatic expansion of the E-Verify system, a computer system designed to ensure employers hire only workers legally present in the United States.

Though some details of the proposal are not yet clear — the bill itself has not been made public — a summary of the legislation provided to The Washington Post indicates that the bill would require companies with more than 5,000 employees to adopt the system within two years. Those with 500 employees would get three years and all employers, including those in agriculture, would need to do so within four years.

The new system will rely more heavily on "photo-matching." The E-Verify system will contain a vast database of photographs, and workers applying for jobs would need to present identification with photographs that match those in the database to prove they are who they say they are and that they are in the country legally.

For non-citizens, that would mean showing a work authorization or green card with a photo. Citizens could show passports or driver's licenses, where states have agreed to provide license information to the E-Verify system. To encourage states to turn over the information, he bill includes $250 million in new funding to reimburse states that choose to take part.

Photo-matching is less expensive for employers — and potentially less invasive-feeling for workers — than finger-printing, which had been considered by senators during their extensive negotiations.

Still, the process already has its critics, which say it amounts to the creation of a new national ID.

"If you take a step back and think about what that means, when this reaches fruition and is completed, you'll have an Internet-accessible system that has everyone’s photo and identifying information in it," said Chris Calabrese, legislative counsel with the ACLU. "You can easily imagine E-Verify becoming the national ID system."

That the system will now be required for employers, even small businesses that might have trouble shouldering the cost, is also a concern, he said. "That's a fundamental shift," he said from the current system, which is largely voluntary.

Jim Harper, director of information studies at the Cato Institute, said photographs are the weakest form of biometric identification.  He predicted that a system that is launched using photos would later be expanded to include other identifiers people might see as more problematic  including iris scans and fingerprints. But he said the "knitting together" of available sources of photographs into a single database is itself a concern.

"I’m standing down on the horizon, waving my arms saying , 'Hey, hey this is a national ID system!" he said. "It's big government at it's worst."

Still, many immigration advocates have shelved their long-time opposition to E-Verify, believing it one piece of a necessary compromise that will also allow a path to citizenship for those now in the country illegally. The system, they believe, is necessary to convince wary conservatives that the nation will not face a new flood of illegal immigrants once a path to citizenship is provided for many who are already here.

Frank Sharry, executive director of America's Voice, said a new system to reduce incentives for employers to hire illegal immigrants — and make it harder for them to do so — is a key piece of any broad-based overhaul of the system.

"It's the essential trade-off: legalization and a path to citizenship, for employer verification," he said. "That is really the heart of every comprehensive immigration reform  package ever put together."