The Senate is debating a bill to legalize the 12 million illegal aliens in the United States. The measure, originally crafted by Sens. John McCain and Edward Kennedy and modified along the way, would first re-label the illegals as guest workers, and then allow them to stay permanently if they met certain conditions. In addition, it would double legal immigration (by adding as many as 1 million new green cards each year), establish a new “temporary” worker program to import at least 400,000 more people yearly, and provide for some limited improvements in enforcement.

If, as is likely, the Senate eventually approves some version of this bill, it would be the counterpart to the bill passed by the House of Representatives in December, which focused on enforcement. That bill establishes a short timetable for all businesses to verify their new hires' Social Security numbers, enhances cooperation between federal immigration authorities and local law enforcement, and calls for expansion of fencing to cover about a third of the border with Mexico.

As with any legislation, the two house of Congress would then try to reconcile their respective bills, in order to come up with a compromise measure to send to the president's desk.

This will be difficult, though, because the differences between the bills are not minor details but rather reflect fundamentally different answers to the question of what to do about the 12 million illegal immigrants who are here.

The Senate approach is based on the false choice between mass roundups of illegals and amnesty. In other words, since we can't deport 12 million people all at once, there's only one alternative – legalization.

In contrast, the House approach rejects both amnesty and legalization and posits a third way, one that might be called “attrition through enforcement” – consistent, comprehensive enforcement of the immigration law (something we have never attempted) designed to reduce the number of new illegal arrivals and persuade a large share of illegals already here to give up and deport themselves. The goal would be a steady decline in the total illegal population, rather than the constant annual increases. Only then would there be a debate on whether some portion of the remaining illegals should be legalized.

This attrition approach is the only workable alternative in any case. Mass deportation is obviously unrealistic: If the 7 million illegal immigrants in the work force (the rest do not work) disappeared overnight, there would be significant disruptions. It's not that the economy ever “needed” these workers, but rather that the economy has accommodated itself to their presence and going cold turkey would be painful. In addition, as the mass illegal alien marches over the past few weeks have shown, mass roundups would likely require the use of military force.

Of course, the effects of the mass-roundup option are irrelevant since the federal government simply doesn't have the capacity to deport so many millions of people over a short period of time. Last year, we deported only about 41,000 aliens from inside the United States (as opposed to those caught at the Mexican border and dumped back across), and that's down from the previous year. Even a tripling or quadrupling of deportations, necessary as that is, can't be the whole solution.

But the Senate's legalization approach is just as impractical. Whether it's done honestly, as in the amnesty passed in 1986, or disguised as a guest-worker program, as in the Senate proposal, a large legalization program is guaranteed to fail. Let us put aside other questions, such as the immorality of rewarding lawbreakers while the law-abiding remain tangled in red tape, and look only at the practical questions: Who would run the program and enforce its requirements? The Department of Homeland Security is choking on immigration, with a backlog of some 4 million applications – it is absurd to suggest that millions of additional applicants could be successfully screened and vetted and tracked by a broken agency that is incapable of meeting even its current responsibilities.

We've tried unrealistic mandates like this before and the results were disastrous. The closest parallel is the 1986 legalization program; about 3 million illegal aliens applied for amnesty, and 90 percent were approved. Hundreds of thousands of successful applications were fraudulent, many presenting fake proof of employment – sometimes as flimsy as a handwritten note on a scrap of paper – and offering ludicrous stories such as, “I've picked watermelons from trees.” One beneficiary of such fraud was Egyptian cab driver Mahmud Abouhalima, who was able to travel to Afghanistan for terrorist training only because of the legal status conferred on him by the amnesty; he then used that training to help lead the first World Trade Center bombing.

Support for either mass expulsion or a guest-worker amnesty is based on a kind of magical thinking, a utopianism that imagines we will be able to resolve this long-brewing policy problem with a single masterstroke.

In contrast, attrition through enforcement requires no magic wand – just consistent, comprehensive application of the immigration law with the tools available, enabling us to back out of a problem that took many years to develop. Such a strategy would have two parts: conventional enforcement at the border and the interior to apprehend and remove illegals (such as the House bill's expanded fencing and federal cooperation with local law enforcement), and what might be called a “firewall” policy, which seeks to prevent illegals from being able to embed themselves in our society. That would involve denying them access to jobs (which is what the House bill's employer verification requirement is about), identification, housing, and in general making it as difficult as possible for an illegal immigrant to live a normal life here, so as to persuade a large number of them to give up and self-deport.

The reason this can work is that there's a lot of churn in the illegal population. According to a 2003 report from the Immigration and Naturalization Service, about 400,000 people are subtracted from the illegal population each year, through deportation, voluntary return, etc. The problem is that the inflow of new illegal aliens swamps this outflow, resulting in an annual increase of the pool of illegals of some half a million each year.

The goal of an attrition strategy would be to decrease the number of new arrivals and increase the number of people here who leave. And there are plenty of relatively recent arrivals among the illegal population who can be induced to leave; a Pew Hispanic Center report recently found that 40 percent of the illegal population has been here less than five years, and fully two-thirds less than 10 years.

And it actually works. To pick just one example, after 9/11, immigration authorities began to enforce some immigration laws against Middle Eastern illegal aliens (and only Middle Eastern illegals). As a consequence, the largest such group, Pakistanis, deported themselves in droves, with a majority of their number leaving the country of their own accord.

Our country faces a choice – the Senate's magical promise of legalization or the House's real-world goal of attrition. Our decision will echo for years into the future.