Hmm, they already have the national ID card.
Just a little something I obtained from aimmigration website faq.
The U.S. Social Security card serves as a national identity document. Canadians will be surprised to find that they will need to recite their social security number frequently. Even their small children will need a number. In fact, your own payroll department will probably be the first to require this number. This page answers the inevitable questions you will receive from your new Canadian employees. We hope it will make your job easier.
The U.S. Social Security card serves as a national identity document. The SS is essentially an inescapable 'work permit' that is foisted on us by our own ignorance and apathy.
I understand it is possible to recind this number but impossible to eradicate it. In seeking employment after that point one will likely be forced to confront potential employers with a 'religious objection' claim and reminders of federal statute on discrimination based on religion.
Not exactly a great way to make an impression. Who wants to hire the next Timothy McVeigh after all?