Opponents have called the measure the most dramatic attempt by a state to police immigration since a 2010 Arizona law — denounced by critics as the "Show Me Your Papers" bill — that was largely struck down by the U.S. Supreme Court. Immigration enforcement is a federal responsibility, and Texas' law is also likely to face swift legal challenges.

The law, which takes effect in March, allows any Texas law enforcement officer to arrest people who are suspected of entering the country illegally. Once in custody, they could either agree to a Texas judge's order to leave the U.S. or be prosecuted on misdemeanor charges of illegal entry. Migrants who don't leave could face arrest again under more serious felony charges.