Pence said (on Hugh Hewitt) they have to leave, get the card, and it's only good for a specified time. At least they will be documented. There is no way all 12 million are being deported anyway.
While it is unlikely that all the current illegals will be deported, what needs to be done is to provide incentives to make them leave the United States voluntarily. Among the measures needed include enforcing existing laws prohibiting employers from hiring illegals (virtual dead letters today), taxing private wire transfers to individuals in foreign countries at a 50% rate, compelling local governments to inform the Feds on the legal status of illegals arrested or ticketed (if it works for seatbelts and DWI, this tactic should work here), prohibiting financial institutions from accepting Mexican "councilar cards" as legitimate IDs, and denying any social services to illegals(if various Catholic bishops and Baptist bigwigs want compassion for illegals, let their churches spring for it, rather than the taxpayers), among other measures.
If the incentive for coming to America is taken away or at least lessened, many of the illegals will return to their native lands.