If you are an unemployed American, Rep. Lamar Smith of Texas has a job waiting for you. We only need to find and deport the 7 million unauthorized immigrants who are now mowing lawns, scrubbing toilets, frying hamburgers, plucking chickens, slaughtering cattle and picking onions and lettuce across America.

To open those jobs for American workers, the Republican chairman of the House Judiciary Committee wants Congress to enact a mandatory, nationwide employment verification program called E‑Verify. The program requires employers to screen all potential new hires through a government database that supposedly will verify if they are eligible to work.

E‑Verify sounds reasonable in principle, but a pilot program has exposed potential problems. A government-commissioned study by Westat found that the system failed to flag more than half of the unauthorized immigrants who applied to work at companies using the system.

The system also exposes too many legal workers to the risk of being falsely denied permission to work. As my Cato Institute colleague Jim Harper concluded in a study of the program, “It would deny a sizable percentage of law-abiding American citizens the ability to work legally. Deemed ineligible by a database, millions each year would go pleading to the Department of Homeland Security and the Social Security Administration for the right to work.”

Isn’t this the kind of intrusive government that tea party members oppose?

Beyond the technical limitations of E‑Verify, Mr. Smith and other supporters are holding out the false hope that an effective verification system will be the key to putting millions of Americans to work. That belief is based on the false assumption that low-skilled immigrants and low-skilled unemployed Americans are interchangeable. They are not.

Immigrant workers are overrepresented in low-paying, unpleasant jobs for the simple reason that not enough Americans want those jobs. The pay, status and working conditions do not match the qualifications and aspirations of the large majority of Americans currently looking for employment in our recovering economy.

In Georgia this summer, a tough new state law aimed at rooting out unauthorized immigrants has created a labor shortage in the farming sector. Gov. Nathan Deal’s plan to replace the immigrants with unemployed parolees so far has not worked out as neatly as planned.

As columnist Jay Bookman noted in the Atlanta Journal-Constitution last month, “The resulting manpower shortage has forced state farmers to leave millions of dollars’ worth of blueberries, onions and other crops unharvested and rotting in the fields. It has also put state officials into something of a panic at the damage they’ve done to Georgia’s largest industry.”

If our politicians actually did succeed in removing millions of unauthorized immigrants from the workforce, middle-class jobs now held by Americans would be in jeopardy. A shortage of low-skilled workers in the agricultural, tourism, food processing, landscaping and other sectors would mean less investment and less employment for managers, accountants, sales reps and other downstream and upstream workers.

A 2009 study for the Cato Institute found that a 28.6 percent reduction in the number of unauthorized low-skilled immigrants in the United States through increased border and interior enforcement actually would cost U.S. households $80 billion a year. The study found that a resulting decline in immigrant labor would mean less investment, more money diverted to smuggler fees and other unproductive uses, and relatively fewer jobs further up the skills ladder.

E‑Verify and other enforcement measures alone cannot solve the problem of illegal immigration. Instead of harassing legal and illegal workers alike, Congress should expand opportunities for legal immigration through a temporary visa program available for low-skilled workers.

Harassing American employers with more government mandates and red tape is no way to create better jobs for American workers.