A Justice Department official said in a letter sent to Abbott that the law was unconstitutional and "contrary to the US commitment of ensuring the processing of noncitizens consistent with the Immigration and Nationality Act."

The law, dubbed SB4 or Senate Bill 4, is part of the state's Operation Lone Star. It goes into effect on March 5.

"SB 4 is preempted and violates the United States Constitution. Accordingly, the United States intends to file suit to enjoin the enforcement of SB 4 unless Texas agrees to refrain from enforcing the law," Principal Deputy Assistant Attorney General Brian M. Boynton writes in the missive, obtained by CBS News. 

SB 4, signed into law by Abbott on Dec. 18, makes illegally entering the US from Mexico a state crime and authorizes police to arrest individuals suspected of crossing the Rio Grande between ports of entry

Migrants caught illegally entering the country would face a Class B misdemeanor charge carrying a punishment of up to six months in jail. Repeat offenders would be subject to a second-degree felony charge punishable by up to 20 years in prison.

SB 4 allows Texas judges to drop charges against migrants if they agree to be deported. 

The new law "intrudes into a field that is occupied by the federal government and is preempted," Boynton argues.

"Indeed, the Supreme Court has confirmed that 'the removal process' must be 'entrusted to the discretion of the Federal Government' because a 'decision on removability' touches 'on foreign relations and must be made with one voice.'"