TOXIC FALLOUT from the Senate's failure to enact immigration reform is drifting over the Northern Virginia suburbs. This month, Prince William County approved a resolution whose purpose is to make life unpleasant for illegal immigrants by denying them services and using local police to hound them. Now neighboring Loudoun County is moving in the same direction, spurred by a member of the Board of Supervisors who contends that immigrant "hordes" are ruining the county. In both cases, the favorite fantasy of elected politicians is that the pressure on undocumented residents will drive them out of the county and into the waiting arms of the feds. Not likely.

Illegal immigrants are in Northern Virginia for the same reason that they are in so many other parts of the country: Their labor is in demand. That's not going to change, unless the powers that be in Prince William and Loudoun have discovered a way to defeat market forces.

In both places supervisors have been quoted as saying that local law enforcement will work with federal officials to deport immigrants who lack the proper papers, including immigrants who might be stopped for traffic violations and other routine offenses. Here's a reality check: U.S. Immigration and Customs Enforcement has a finite number (27,500) of detention beds nationwide. They are apportioned among asylum seekers, fugitives awaiting removal, border-crossers, ex-cons, those who have committed violent crimes and others. Given limited resources, ICE's priority is to detain people who are threats to national security and public safety, which leaves little room for the undocumented gardeners, home builders and restaurant workers whose presence in Northern Virginia has so annoyed local politicians.

The truth is, ICE simply does not have the time, personnel or capacity to do what local politicians want: return vast numbers of illegal immigrants to their country of origin. The fact that supervisors in Prince William and Loudoun say they are forcing the immigrants out will not make it so. Prince William's police chief has said publicly that ICE will accept the handover of a very limited number of undocumented aliens from the county each month. An educated guess is that more undocumented immigrants -- possibly a lot more -- will arrive each month than can realistically be removed.

Local politicians are under growing pressure from constituents whose neighborhoods are afflicted by flophouses, poor sanitation, cars parked on front yards, loiterers and other ills they associate with illegal immigrants. The complaints are understandable -- nobody wants to see their neighborhood go to seed -- and counties are within their rights to urge tougher enforcement if laws are broken. But by singling out illegal immigrants, local politicians are contributing to what is becoming a poisonous, increasingly nativist atmosphere that will infect relations with Hispanics generally. After all, no one can distinguish between legal and illegal immigrants at a glance.

American history has had its ugly episodes of foreigner-bashing, and the rhetoric this time is reminiscent of the xenophobia that greeted previous waves of Jewish, Italian, Irish and Chinese immigration. The legitimate cause of safeguarding neighborhoods must not be twisted into a justification for implanting new roots of nativism.