Proponents of the latest Senate effort to change the nation's immigration laws emphasize border security. Indeed, the very title of the bill, the "Secure Borders Economic Opportunity and Immigration Reform Act of 2007," says so.

This reflects public opinion. Surveys over the past two years consistently show that over two-thirds of our citizens believe border security should be the first priority in any immigration reform plan.

Unfortunately, the Senate bill offers little border security protection beyond what is already provided in existing federal laws. Furthermore, the new proposal creates a set of conditions virtually guaranteed to spark a surge of illegal aliens crossing our borders the moment the new provisions go into effect.

Let's look at the "Secure Borders" provisions of the bill. With recent, "strengthening" amendments, the bill now calls for 20,000 additional border patrol agents, expanded detention facilities to accommodate 31,500 detainees, at least 300 miles of vehicle barriers and 370 miles of fencing, 105 ground-based radar and camera towers, and four unmanned aerial vehicles deployed along the southern border.

All of these measures are worthwhile, but they fail to go very far beyond what Congress has already accomplished in existing statutes, much of which is currently in progress. For example, the Secure Fence Act of 2006 requires the Department of Homeland Security (DHS) to hire 18,000 border patrol agents by the end of 2008, and the agency has already hired some 15,000 of them. That act also required 700 miles of fence along our 1,951-mile southern border, more than the combination of fencing and vehicle barriers set forth in the new legislation.

Likewise, the 2004 Intelligence Reform and Terrorism Prevention Act requires the Immigration and Customs Enforcement agency to have 43,000 new detention bed spaces available by the end of this year, far more than the number specified in the new bill.

But, say the bill's proponents, the new proposal requires that the Secretary of Homeland Security establish and demonstrate "operational control of 100 percent of the international land border between the United States and Mexico" before other provisions granting legalization to illegal aliens go into effect. That language sounds tough, but the bill contains no definition of "operational control" and carries no definable, measurable performance standard. Last year's Secure Fence Act defined operational control for the purpose of that law as "the prevention of all unlawful entries into the United States."

The provisions described above would do no harm, though they would yield little additional border security. But a companion provision of the bill would negate its positive aspects and actually encourage illegal immigration during a window created by the establishment of "probationary" Z visas.

Within 180 days after enactment of the bill, the government would have to begin accepting illegal aliens' applications for Z visas. Under this new category of permits, any illegal alien can apply for legalization by showing some evidence (for example, an affidavit signed by a non-relative plus a second document such as a pre-2007 bank statement or lease, etc.) that he has been residing in this country since before January 1 of this year.

In each case, DHS must issue a probationary visa within one business day of the application, unless it discovers a reason for disqualification within that 24-hour period.

Under the proposal, issuing a probationary Z visa takes precedence over applying the regular immigration laws that govern illegal entry and presence, so that applicants could not be deported or subjected to other enforcement procedures -- even if they had been apprehended before the legislation is enacted. Thus millions of illegal aliens would quickly be allowed to remain in the U.S. with legal status.

Our security system would be overwhelmed. The task of carefully checking each applicant within one business day would be virtually impossible. If government agencies could find no basis for denying the visa application of 9/11 terrorist Mohammed Atta six months after he flew into the North Tower in New York City, DHS is unlikely to uncover many bad actors within a 24-hour time limit.

Moreover, the minimal documentation needed to support a probationary visa application is immediately susceptible to fraud. The extensive industry that now provides illegal aliens with false Social Security cards and other fraudulent documents will shift into high gear, producing the far less secure papers acceptable as "proof" under this proposal. Terrorists, drug dealers and other criminals will eagerly -- and easily -- exploit a security system that will be inundated by millions of applications demanding virtually immediate clearance.

There's another, equally predictable result of the bill: a massive wave of new illegal aliens entering the country. The proposed legislation provides an extendable 18-month period in which people may apply for a probationary Z visa. This window of opportunity will attract an enormous number of new, illegal entrants who need only pick up two false documents to join the line of 12 million applicants already in the country. After skating through the 24-hour review, they will receive legal status in the U.S., free from interference by immigration-enforcement authorities.

Yes, the bill does require that, once security triggers are met, probationary Z visas must be exchanged for permanent Z visas, a process that includes more detailed checks and other requirements. And, yes, immigration authorities can cancel probationary Z visas if security problems surface or they find that they have been obtained by fraud. But these provisions don't change the fact that the bill will enable untold millions of people who entered illegally to legally live, work and move freely about the country. As a practical matter, that makes such enforcement virtually impossible.

Indeed, we can count on a surge of people trying to slip in during the probationary visa window, before the border security measures are fully implemented. That's what happened in 1986, when amnesty sparked a tidal wave of fraudulent applications and a huge increase in illegal immigrants. As before, many more want to get the "deal" and will try to melt into this amnesty or get in while they can in the hope that our nation will offer another one in the future.

Beyond these security problems, the proposal contains provisions that threaten the privacy of every American worker, place new burdens on nearly every business, and greatly expand government intrusion into our work places.

How? Title III of the bill would create an enormous new program -- the Employment Eligibility Verification System (EEVS) -- and require nearly all businesses, large and small, to participate. EEVS imposes costly compliance and record-keeping requirements on business owners. Moreover, the proposal would require all workers -- citizens, permanent residents and others lawfully employed -- to prove they are eligible to work. They will have to get official confirmation from the DHS just to keep their current jobs, and again every time in the future they apply for new jobs.

The employment verification system hinges on the government assembling -- accurately and securely -- a vast amount of information on millions upon millions of workers and businesses into a single database. Available to numerous federal, state and local agencies, the database would be subject to the same kind of abuse, improper handling, or even identity theft, we've seen repeatedly when federal agencies get their hands on similar, "confidential" data. How ironic that legislation that benefits illegal aliens would cause major hardships for law-abiding workers and business owners.

Other defects abound. But the serious problems described above, and the experience of 1986, should convince lawmakers that this bill will not resolve our illegal immigration problems, even as it diminishes the liberty and privacy of working citizens.

This isn't the 1986 amnesty deal all over again. It's worse.

Edwin Meese III is a Distinguished Fellow at The Heritage Foundation, where he holds the Ronald Reagan Chair in Public Policy.