While immigration has been historically cast as a federal responsibility, state and city governments have seized upon the U.S. government's immigration deadlock by stepping in to fill the void. In the first eight months of 2007, local jurisdictions passed more than 170 immigration-related laws - more than doubling the number from 2006.

Not surprisingly, San Francisco has entered the fray. Having already declared the city a symbolic haven for undocumented immigrants, the Board of Supervisors recently proposed issuing municipal ID cards to all city residents. These cards would help undocumented people apply for jobs, access city services and open bank accounts - all to ensure that the immigrant population is employed, educated, willing to report crimes and treated with dignity.

Yet lurking behind the ID proposal is a fundamental constitutional issue. Opponents of the plan argue that the city's proposal violates the U.S. Constitution because it frustrates principles of federal supremacy and power with regard to immigration matters.

Although the opposition's claim has legal merit, it is not without irony. Just a few months ago, pro-immigrant forces used the same argument to persuade a federal court to overturn a restrictive ordinance against illegal immigrants in Hazelton, Pa. That measure would have denied such immigrants employment, social services and housing.

Political majorities in San Francisco and Hazelton share the common belief that municipalities can and should deal with their shadow populations of undocumented people. They just disagree on what their local communities should do with that power. Likewise, dissenters in both cities share the same legal strategy, even if they are pursuing different agendas.

I want to suggest that rather than opt for a zero-sum game of equal and opposite legal claims, both sides might consider focusing on their shared faith in local action. Perhaps we are ready as a nation, in the absence of clear, consistent, and specific federal mandates and enforcement, to consider whether cities should be permitted to manifest their diverse viewpoints on the issue of undocumented immigrants.

Rather than reflexively resort to expansive claims of federal power and competency, it's time to initiate a conversation, asking ourselves whether we should allow towns such as Hazelton, which have chosen to express xenophobic, law-and-order impulses, to coexist with cities such as San Francisco, which believe that charitable treatment of undocumented people will prove healthier in the long run.

The Constitution not only mandates that federal law remain supreme over state law, it also grants power to Congress to establish a "uniform rule of Naturalization." This plainly requires state and local subordination to specific federal immigration and naturalization laws. However, the broader claim that the Constitution also commands state and local silence on any aspect of the lives of undocumented immigrants, even in the absence of clear federal mandates, stands on shakier ground.

Border enforcement and naturalization procedures are national concerns deserving of nationwide solutions. In contrast, once undocumented people have lived and worked within the nation's borders, municipality-specific lawmaking may be the better response. And, with Congress lacking the motivation and political will to provide a coherent, sensible solution, local jurisdictions have energetically entered the immigration debate.

It is possible that the diverse laws and resulting disparate treatment of undocumented immigrants across regions and states may be so unwieldy that they finally force Congress to compromise, using the localities' experiences as a guide.

In the event of continued federal inaction, at least undocumented immigrants will face more certainty in the form of explicit municipal policies, even if it means invidious certainty.

As a San Francisco resident, I support the city's proposed municipal ID ordinance, because it provides assurance, dignity and opportunity to many hardworking members of our society. As a citizen of the United States, I believe that our constitutional principles allow our city to act compassionately, even as it provides others the option not to.

Although some localities will shut the door and plant a no-trespassing sign, others will provide a sanctuary. Given our nation's economic dependency on the labor and skills of undocumented and documented immigrants, the latter group is likely to flourish.