To those who would crack down on illegal immigrants, it seems an obvious strategy: Have tens of thousands of local law-enforcement officers carry out federal immigration law by checking the status of people they stop or arrest.

It turns out to be more complicated than that, but the number of communities endorsing the idea – including, most recently, Virginia's Prince William County – has been gradually rising and is expected to jump even higher in the months ahead.

"This is something we're going to see a lot of now," says law professor Dave Martin of the University of Virginia in Charlottesville. He says public frustration over high levels of illegal immigration and Congress's failure to agree on reforms is spurring the reaction.

In the past, cities that welcomed diversity and new immigrants made a point of refusing to let their police officers help federal agents identify people who might be in the US illegally. Others worried that their departments would be slapped with harassment or racial-profiling lawsuits if they became involved in enforcing US immigration laws.

But when political leaders in Prince William County saw national reform legislation falter last month in the Senate, they approved their own immigration-reform resolution that, among other things, would give local police a shot at enforcement.

To that end, the Board of Supervisors voted unanimously on July 10 to allow county police officers the authority to check the citizenship status of anyone they've stopped or arrested whom they have "probable cause" to believe is in the US illegally. The county board has yet to define "probable cause," but board chairman Corey Stewart says it may be based on whether a person has a driver's license.

The county "has reached a boiling point," says Mr. Stewart. An influx of illegal immigrants over the past four years has led to overcrowded houses and schools, overstretched public services, and a rising problem with gangs, he says.

With its vibrant economy and healthy job market, Prince William County, located about 30 miles outside the nation's capital, has by one estimate attracted 40,000 illegal residents – about 10 percent of the total population.

Even as other localities consider taking similar steps, cautionary notes persist. Regardless of how county officials define probable cause, racial and ethnic profiling will be "unavoidable," says Joan Friedland, immigration policy director of the National Immigration Law Center, an immigrant rights advocacy group.

Using an approach that takes race, ethnicity, or language into account is what landed Hazleton, Pa. – which passed an ordinance in July 2006 that punishes landlords for renting to illegal immigrants and employers for hiring them – in court, following challenges by the American Civil Liberties Union (ACLU) and other immigrant-rights groups. A decision in the case is expected soon.

One way communities can reduce their legal risk is by entering into an agreement with the Department of Homeland Security (DHS), says Cristina Rodriguez, a law professor at New York University. The agreement gives local police officers authority to enforce federal immigration law after they receive training in cultural sensitivity and in techniques to avoid racial profiling.

Though Prince William County's resolution calls for such an agreement, it will also require many police officers and county officials who do not have the federal training to make citizenship determinations, Professor Rodriguez notes.

The county already has such an agreement with DHS: Local officers who staff county detention centers screen all incoming individuals for legal status. Under the new resolution, the county plans to expand that ability to general police officers, so they can screen anyone they detain and suspect of being illegal. Every month, Immigration and Customs Enforcement (ICE) allows the county to send it as many as 40 persons whom county officers determine are illegal for deportation. Stewart says he hopes to see the number increase.

Twenty-one state and local law-enforcement agencies have already struck such agreements with DHS, and about 75 others have submitted applications. The program was first authorized in 1996, but most agreements have been signed in the past 18 months, says an ICE spokesman.

A key question is whether such enforcement action will result in more deportations and deter future illegal immigration.

Rodriguez, for one, insists it will not accomplish such aims. "The symbolism could make some people leave, but the overwhelming factor is the economic factor, and if there are jobs, the population will remain," she says.

But in Arizona's Maricopa County, a major thoroughfare for border crossers, the sheriff's office sees it differently. It entered into a pact with DHS nine months ago, and its website says deputies acting under federal authority have so far identified 182 arrested persons as illegal immigrants. The program has been successful so far, says Paul Chagolla, spokesman for the Maricopa County sheriff's office.

Though ICE covers the cost of training and some of the extra enforcement-related expenses, Stewart expects that the costs of Prince William County's immigration overhaul eventually will run into the millions. The resolution also directs county agencies to determine which public services can legally be denied to illegal immigrants. Critics like Ms. Friedland suggest that move will deter such immigrants from using critical services such as emergency medical care that they cannot be denied under law out of fear of deportation.

So far, the ACLU of Virginia has no plans to challenge Prince William County's resolution. That could change, says executive director Kent Willis, after its details become clear.