They are leaving. Illegal immigrants, that is.

Analysts from both ends of the immigration debate, from the Center for Immigration Studies to the Pew Hispanic Center, agree. The "unlawfully present" population in the United States has shrunk - and it's getting smaller.

According to Pew, there has been a drop in the annual flow of people illegally entering the country since 2005. And the numbers of those already here is going down. It peaked at 12.4 million in 2006 and is down by about 1 million now.

What the analysts don't agree on is why. Good enough data simply aren't available to answer the question. There are a number of possibilities, though.

It could be simple economics, with fewer jobs translating into fewer illegal immigrants. Historically, whenever the U.S. economy has shrunk, fewer workers risk coming north to seek employment. They find economic opportunities at home or migrate to alternate destinations.

Homeland Security Secretary Michael Chertoff wants part of the credit, too, citing "tighter border security, a significant expansion of the Border Patrol, the deployment of new technology, and increased interior enforcement are having an undeniable impact."

Contrarians, however, are quick to deny that. They seem eager to paint enforcement not only as ineffective but evil.

According to Tom Barry of the Center for International Policy's "Americas Policy program," enforcing the "rule of law" is little more than a fig leaf for some anti-immigration types - one that allows its advocates to appear "ostensibly separate from the nativists, economic populists, and white supremacists that make up much of the base of the movement."

This is a remarkable claim. No doubt many in Congress - including Sens. Ted Kennedy and John McCain, who pushed for a comprehensive reform that included a number of border security and workplace enforcement measures (as well as all in Congress who voted to build a fence to help stop illegal entry from Mexico) - would be stunned to find out they're just fronting for the Ku Klux Klan.

Demonizing law enforcement and border security seems like an effort to poison the well for promoting serious further reforms. There are already whisperings that some policy-makers will try to roll back enforcement innovations such as the compacts between the Department of Homeland Security and state and local law enforcement.

The result of such efforts is that both Congress and the American people will become more reluctant to embrace any kind of immigration reform. Instead, they will suspect that the administration is offering "86" all over again.

In 1986, Congress passed comprehensive reform that granted a general amnesty and promised more workplace enforcement and better border security. At the time, the number of illegals was about 3 million. Now it is more than three times that. Granting amnesty just encouraged more illegal immigration, while Washington did little to secure the border or enforce the law.

Failure to enforce the law, combined with an amnesty that brushed aside the rule of law (along with strong economic growth for more than a quarter of a century) created the crisis that we have today - a crisis that needs to be solved.

Presumably, no one thinks a depression is the answer (if there are no jobs, there would be no illegal workers). We need to get the economy growing again. As the economy recovers, if we want to continue to reduce illegal immigration, then we are going to have to have border control, enforce the law (not grant amnesty), and institute reforms that help provide sufficient legal avenues for employers to get the workers they need to help the economy continue to grow and prosper.

And if we can do it with more workers being "lawfully present," so much the better.

James Jay Carafano, a senior research fellow in national security issues at The Heritage Foundation, is the author of "G.I. Ingenuity."