The move, which could expose unregistered migrants to criminal prosecution, represents a drastic escalation of the administration’s efforts to push millions of immigrants to leave on their own.

But lawyers and immigrant advocates warned that the administration’s plan was on shaky legal ground and that some immigrants may be afraid to comply.

This week, the Trump administration announced that it will ramp up enforcement of a relatively obscure legal provision that requires undocumented immigrants in the United States to register with the government.