By Stacy Washington

Imagine if President Trump's much-touted wall cost nothing to build. And if it deterred not only illegal border crossers, but also those who legally come to the United States and then overstay their visas. And if it could be erected almost instantaneously.

Impossible? Actually, that wall already exists - electronically.

E-Verify is a free, online resource that empowers employers to check whether employees are eligible to work in the United States. It can ferret out illegal workers in seconds.

Requiring all businesses to use E-Verify would drastically reduce illegal immigration by turning off the jobs magnet that attracts illegal laborers. That would protect American workers and boost wages. And unlike most Washington regulations, it would do so without burdening taxpayers and businesses or infringing on citizens' civil liberties.

The Department of Homeland Security administers E-Verify. The site cross-references employees' I-9 work authorization forms against their Social Security numbers, visas, and other government records. In a matter of seconds, the system either green-lights a hire or issues a "tentative nonconfirmation," which lets employers know that the person might not be authorized to work.

The system is overwhelmingly accurate. From October 2015 to June 2016, E-Verify processed about 24 million cases. Of those, nearly 99 percent were green-lit immediately. Barely 1 percent resulted in tentative nonconfirmations - and 86 percent of those tentative nonconfirmations ultimately resulted in final nonconfirmations after further review.

In other words, the vast majority of people who received tentative nonconfirmation notices truly weren't authorized to work, and the system caught them. Further, employers who use E-Verify are dissuading illegal workers from applying in the first place.

Many states - such as New Mexico, Georgia, and North Carolina - already require all employers to use E-Verify. Many other businesses use the service voluntarily, since it prevents them from unwittingly employing illegal workers. More than 600,000 employers have enrolled in E-Verify, and more than 1,000 employers join each week.

Some corporate groups have warned that requiring the use of E-Verify would place a burden on businesses. This is simply not true. E-Verify provides an initial result in five seconds or less, and it requires virtually no additional equipment or training. Three-quarters of businesses say that using the system would cost them absolutely nothing.

Other critics, such as the American Civil Liberties Union and the Cato Institute, worry that mandating E-Verify would set the United States on a slippery slope to a national ID system. Such fears are unfounded. E-Verify relies on existing legal documents already required by the I-9 form all employees must fill out, and the law already prohibits hiring illegal workers. Making the system mandatory would simply improve enforcement of laws already on the books.

Mandating E-Verify would curb illegal immigration far more effectively than even the highest border wall could. That's because 40 percent of illegal immigrants arrive legally and then overstay their visas. Making it nearly impossible to hold a job without legitimate work papers would compel many illegal workers to return home and discourage others from illegally immigrating in the first place.

Ridding the labor pool of illegal workers would spur wage growth, especially for the lesser-skilled American workers who struggle most in today's economy. Immigration reduces these workers' wages by up to $1,500 a year, according to Harvard economist George Borjas.

Blue-collar American workers aren't the only ones who would benefit, though. Illegal immigration affects all sectors of the U.S. economy. One in eight illegal immigrants in the United States holds a white-collar job, according to a recent Pew Research Center study.

E-Verify will also level the playing field for businesses. It's deeply unfair that firms who hire illegal workers and pay them artificially low wages can undercut ethical companies that hire legal workers and pay them fairly.

Making E-Verify mandatory for all employers is the quickest, most efficient way to deter illegal immigration. It would boost wages and open up job opportunities for American citizens and legal immigrants. And since this electronic wall is already in place, nobody would have to pay for it. Not even Mexico.

Stacy Washington hosts the nationally syndicated radio program "Stacy on the Right," based in St. Louis.