If you hang everyone, the old saying goes, you will catch some guilty people. That adage points to the fatal flaw of an employment-verification program tucked into several recent immigration-reform proposals in Congress.

The E-Verify program—which several states are experimenting with, but which would become mandatory nationwide under proposed new law—targets every employee that a business hires, in the hope of weeding out a few undocumented immigrants from the workforce. In the process, E-Verify erects dangerous hurdles to employment for legal workers and degrades the privacy of working Americans.