Immigrant rights advocates are set to bring one of the first legal challenges against a city that allegedly contravened the California Values Act.
Xavier Becerra joined Julián Castro and Beto O'Rourke in calling to end criminal charges for those who enter the country illegally.
Challengers to Trump's national-emergency declaration can use his own words against him.
The letter comes one week after the EPA announced that it would not enforce a 2016 regulation that limits the manufacture of diesel freight trucks refurbished with older engines.
But analysts anticipate the war between Sacramento and Washington will go to the Supreme Court.
In the fight over immigration policy, the state and its municipal opponents are gearing up for a series of legal battles.