Gavin Newsom is following up with his threat to sue President Trump over the deployment of the National Guard to Los Angeles … because the California Governor just filed a case against POTUS over it.

Newsom and the State of California are suing Trump, Defense Secretary Pete Hegseth and the U.S. Department of Defense over what Newsom says are “illegal actions” to deploy members of the California National Guard.

Trump sent the National Guard into L.A. Saturday after protests erupted over federal immigration raids … a move Newsom says is unconstitutional.

In the lawsuit, obtained by TMZ, Newsom claims Trump had no authority to deploy the California National Guard … Newsom says that the statute the president invoked clearly requires that action to go through the governor.

Newsom claims Trump used a protest that local authorities had under control as a way to make an “unprecedented power grab.”

As we reported … Newsom on Sunday ripped Trump, calling the Prez a “stone cold liar” who wants to start a civil war over immigration.