The Arizona legislature has passed a law that makes it a misdemeanor to be in the United States without proper paperwork, and requires local police to determine the immigration status of anyone they reasonably suspect of being here unlawfully.

Arizona Governor Jan Brewer, a Republican, is expected to sign the bill, several area news sources report, and GOP legislators are celebrating its passage. "Illegal immigration brings crime, kidnapping, drugs—drains our government services," Rep. John Kavanagh, a Republican, told the Los Angeles Times. "Nobody can stand on the sidelines and not take part in this battle."

Arizona must instead find a way to address aspects of its illegal-immigration problem without subjecting a subset of its citizens to de facto ethnicity-based harassment.

Elected officials in Arizona are right to worry about illegal immigration. Its costs are disproportionately born by the state's poorest citizens, who also benefit least from the cheaper services provided by immigrant labor. Area schools suffer when an unpredictable number of students with language barriers enroll each semester. And increasingly violent drug cartels in Mexico and a California border heavily fortified against illegal entrants mean that folks along the Grand Canyon State's 389 mile southern border are particularly vulnerable to international lawlessness.

It is nevertheless imprudent to fight illegal immigration with laws targeting otherwise law-abiding residents with arrest. The conventional objection is that doing so discourages people in immigrant neighborhoods from working with police: Undocumented residents won't act as witnesses or good Samaritans or report being victimized by crime if calling 911 might well result in their deportation.

These are sound objections.

As important is the effect this law is bound to have on the 1.5 million-plus Hispanics in Arizona who are American citizens or legal aliens. Put simply, these people are going to be routinely asked by police for proof of citizenship, especially in a state where racial profiling is already a problem. Witness Maricopa County, Arizona, where the odious Sheriff Joe Arapaio presides over a police force sued multiple times for targeting Hispanics, many of them American born—and where police have already been sanctioned by a judge for destroying evidence related to a profiling lawsuit. The Mexican consulate in Phoenix is already reminding its constituents of their rights if they are harassed.

• Terry Greene Sterling: A Crazy Immigration LawMy personal preference is for an increase in the number of people legally allowed into the United States, so that more residents in places like Arizona operate within the legal system, an increased law-enforcement presence along parts of the border unprotected by an effective fence, and sanctions against employers who hire undocumented workers. Of course, these are federal prerogatives, and one can sympathize with states like Arizona that suffer disproportionate costs associated with illegal immigration, yet are unable to address its root causes through legislation or voter initiatives.

A state law that can only result in the mistreatment of Hispanic Americans is nevertheless an unjustifiable response. Arizona must instead find a way to address aspects of its illegal immigration problem without subjecting a subset of its citizens to de facto ethnicity-based harassment (it is implausible to think that municipal police forces can be trusted with the discretion to do immigration checks in an ethnically mixed region—it's a recipe for misconduct, corruption, and racial tension).

The best solution I can think of is a policy of checking the immigration status of everyone convicted of a crime in Arizona. On serving their sentence, citizens would be released into the United States, and non-citizens would be deported.

This policy has the benefit of being race neutral in the inconvenience it imposes on those being investigated—every criminal, black, white, Hispanic or Asian, would be checked. Deportations would be automatically targeted at lawbreakers, as opposed to illegal immigrants who merely seek a place where they can work hard to support their families. And asking jailers to determine immigration status instead of police officers means that the latter could continue interacting with illegal immigrants while fighting criminals, gathering evidence, and policing neighborhoods.

Hispanic advocacy organizations may object even to my compromise policy, but they shouldn't: If these illegal immigrant convicts return to their country of origin rather than their ZIP Code of residence, law-abiding illegal immigrants will benefit as much as anyone.

Conor Friedersdorf blogs at True/Slant and The American Scene. Follow him on Twitter at Conor64.