Adam Serwer explains:
[M]ake no mistake, when Romney is discussing "self-deportation," he's talking about creating a United States where parents are afraid to register their kids for school or get them immunized because they might be asked for proof of citizenship. He's talking about the type of country where local police can demand your immigration status based on mere suspicion that you don't belong around here. "Self-deportation" is just a cleaner, less cruel-sounding way of endorsing harsh, coercive government polices in order to make life for unauthorized immigrants so unbearable that they have no choice but to find some way to leave. The human cost of such an approach, let alone what it might do to American society, is viewed as a price worth paying.