Illegal aliens cannot get more-than-casual employment - which they obviously want - without a Social Security number. However, illegal aliens cannot obtain valid Social Security numbers, citizenship or a green card is required.
So, many illegals have used fake SSNs, basically those of other people, to apply for and get work. This, it turns out, is identity theft which is a Federal felony.
President Trump says he only plans to deport criminals. It turns out that basically includes everybody without valid documents working with a fake or stolen SSN.
The men cutting meat at a packing plant, the women changing beds at a motel, they are all felons if their SSNs are invalid or "borrowed." Such individuals may number in the millions.
My wife's elderly aunt, long retired and drawing Social Security benefits, has learned someone out there is fraudulently using her SSN. She has reported it but the Social Security Administration has no mechanism by which to police such misappropriations.
Imagine what happens when millions of the undocumented employed reach retirement age. They cannot get Social Security because someone is already receiving the benefits associated with the SSN they've been using. They'll end up on welfare, and we'll pay for it with our taxes.