ILLEGAL IMMIGRATION, PRO AND CON. Virginia Postrel expresses a preference for an immigration policy that can separate the dishwashers from the terrorists. I doubt that anyone would gainsay that. But designing such a policy is harder ... does anyone even have a guess how many terrorists might be masquerading as dishwashers? She links to a Dallas Morning News editorial (I'm going to quote freely before this goes behind the firewall) that pursues multiple goals.

The Judiciary bill would offer the 11 million to 12 million illegal immigrants in the U.S. a chance to earn citizenship. They would have to show they have a job, pay a fine, pass a background check, get their taxes in order and go to the back of the line to apply for citizenship.

Let us repeat that last part, because it often gets misread as amnesty. Illegal immigrants who do the above would not cut in front of anyone else applying for citizenship. They'd have to line up behind everyone else seeking the same citizenship that many legal Americans' own immigrant ancestors sought.

Operationally, it is an amnesty. Irregular workers become regular workers and pay taxes. Citizenship and voting rights are for another paper. Here's a harder problem.

The nation's security is far stronger if we know who's here to frame houses, change linens, bus tables and build microchips – and who shouldn't be here to profit from true criminal activity or worse.

Although now you have a Bayesian inference problem. Does anyone have a subjective probability that a putative dishwasher is a sleeper agent?

There's also a resource allocation problem.

A citizenship plan also makes sense because we simply can't enforce existing laws and send the illegals back where they came from.

That refrain, a staple among some readers and on talk shows, would require rounding up a population the size of Ohio. We don't have enough border agents, police officers, holding cells, judges, courtrooms or buses to make that happen.

Paying several billion dollars to do something of that scale would be fiscally imprudent.

What are the opportunity costs?

The committee would be best served to go with the idea presented by Sens. John McCain and Ted Kennedy, who would allow as many as 400,000 foreign workers each year to receive three-year work visas. They could earn two three-year visas before returning home or earning U.S. citizenship.

That number matches the estimated annual flow of illegal immigrants. If we take those workers and give them forge-proof, biometric ID cards, we ease the burden on law enforcement agents. They will know immediately who's here legally and who's not.

Is there such a thing as a forge-proof identity card? On the other hand, if the authorities knew who was here legally, wouldn't it be straightforward enough to deport the equivalent of the population of Ohio?

Illegal immigrants make up about 5 percent of our workforce. Yank all of them out of the economy, and we have a serious financial problem. You can't punch that big a hole in the workforce without upending industries, farms, manufacturers and the hospitality industry – not in today's nearly full employment economy.

Yes, fewer American high school dropouts have jobs today, but even the authors of a recent study about this admit there is no evidence that illegal immigrants have their jobs. It's more likely that many jobs would go overseas or disappear altogether if immigrants didn't do them.

Um, how big is the job churn in a "nearly full employment economy" and how does one send Vermont's cows or Las Vegas's hotel rooms to India?

In related commentary, InstaPundit received a favorable report on Chicago's illegal immigrants' rally and links to a local source that had a less favorable impression.