Although the commission shied away from directly making the controversial call in its final report, "We did recommend national standards for drivers' licenses," Vice Chairman Lee Hamilton told the Senate Commerce, Science and Transportation Committee.
"Over time," he told committee Chairman Sen. John McCain, R-Ariz., "that might lead to a national ID card."
"Isn't that a fundamental issue that we're going to have to address as a nation?" McCain asked Commission Chairman Tom Kean.
Kean replied that the commission had judged biometric screening -- using techniques like fingerprinting or retinal scanning -- to be "a little less intrusive," but acknowledged "A national ID card would be another way to do it."
The American Civil Liberties Union said in a statement Tuesday that federal standards for drivers' licenses constituted a "backdoor attempt to create a national ID-card system and a serious threat to privacy, liberty and safety."
They said the idea "would provide a new tool for racial, religious or ethnic profiling and would lead to far more illegal discrimination."
The American Association of Motor Vehicle Administrators, which represents state DMVs, welcomed the proposal. "We concur with the commission's recommendation that the security of both the issuance process for the state-issued driver's license and the document itself need to be strengthened," said the group's president, Linda Lewis.
But the emergence of the license as what the association calls the "identification document of choice throughout North America" has brought with it problems.
A number of state governments have faced controversy since Sept. 11, 2001, about the so-called legal-presence requirement, a law that requires applicants for a license to prove their U.S. citizenship or their right to reside in the United States.
Advocates say such rules are essential to maintain the integrity of the de facto national ID document, but immigrants' rights advocates counter that trying to exclude undocumented migrants turns "motor vehicles bureaus into immigration agents -- without additional funds or adequate training," in the words of the ACLU.
Motor vehicle administrators also have reservations about the legal presence requirement. "Our initials are D-M-V, not I-N-S," said American Association of Motor Vehicle Administrators spokesman Jason King -- referring to the acronym of the Immigration and Naturalization Service, which was absorbed into the new Department of Homeland Security last year.
"We are the experts in driver licensing, not immigration."
Tuesday, Hamilton said he thought McCain's call for a debate on the issue of a national ID was right on the mark.
"The American public is becoming more and more agreeable to intrusiveness in order to protect themselves against terrorist attacks," he said, adding that the idea was nonetheless still controversial.
But in its final report last month, the commission, formally known as the National Commission on Terrorist Attacks Upon the United States, proposed changes to the nation's identity regime more radical than federal standards for drivers' licenses.
The report recommended a seamless biometric border so that everyone entering or leaving the country, whatever their nationality, would have their identity biometrically confirmed.
Currently, under the U.S.-VISIT program only one category of travelers -- those foreigners holding visas -- is subjected to such a check, and then only at airports.
The Department for Homeland Security, which runs U.S.-VISIT, plans to extend the system to cover foreigners from the 27 visa-waiver nations who arrive without visas and to roll it out gradually at border crossings beginning later this year.
But even when U.S.-VISIT is complete at the end of 2005, U.S. citizens will still be able to enter the country without a biometric identity check. The State Department recently announced that, by the end of that year, it will begin issuing U.S. passports with a single biometric identifier -- a digitized photograph that can be checked with facial-recognition software.
At Monday's hearing, Sen. George Allen, R-Va., questioned the choice of facial recognition as the biometric, saying it had a failure rate as high as 50 percent.
State Department officials point out that the digital photograph is the international standard for passport biometrics, agreed by the International Civil Aviation Organization.
But advocates of biometric security for travel documents say that this was simply the path of least resistance -- people are used to submitting their photographs already.
"The harmonization with other countries is important," acknowledged Allen. "But is it so important to disregard the fingerprint-technology option, which I believe is superior to the State Department's current plan?"
A State Department official pointed out that collecting photographs from passport applicants and digitizing them was easy, compared with having to install equipment to do fingerprint scans at the 6,000-plus facilities -- including post offices and court houses -- that currently accept passport applications.
"First time applicants, or those whose passports expired more than five years ago, have to apply in person," the official said. "Installing fingerprint stations at all those locations would be a serious logistical burden."
Privacy advocates also point out that, since the passports and the data encoded on them will be machine readable, using fingerprints as a biometric would effectively be requiring U.S. citizens to hand a permanent digital record of their prints to the government of any foreign country they visited.
The commission further advocated that the biometric border be fully integrated with an internal system of checkpoints, controlling access to the nation's critical infrastructure and transport system, as well as federal buildings.
"That could be a recipe for a system of internal controls that would treat people traveling within the United States in the same way it treats those crossing its borders," cautioned Greg Nojeim, associate director of the ACLU's Washington office.
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