"Federal law prohibits the use of active-duty service members for law enforcement inside the U.S., unless specifically authorized by Congress. But over the past 12 years, presidents have twice sent National Guard troops to the border to bolster security and assist with surveillance and other support.
Nielsen said the effort would be similar to a 2006 operation in which President George W. Bush deployed troops to help U.S. Customs and Border Protection personnel with non-law enforcement duties while additional border agents were hired and trained. President Barack Obama also sent about 1,200 troops in 2010 to beef up efforts against drug smuggling and illegal immigration.
Nielsen said her department had developed a list of locations where it would like assistance on things like aerial surveillance and other support, and was discussing with the governors how to facilitate the plans. She declined to say how many personnel would be needed or how much the operation would cost, but she insisted, “It will be as many as is needed to fill the gaps that we have today.”" realclearpolitics
(159th Infantry Regiment CANG)
The Posse Comitatus Act forbids the US of federal armed forces in internal law enforcement operations in the US. There is an exception to this prohibition in the Insurrection Act for the obvious reason that insurrection would by definition be internal to the US.
The US National Guard is that portion of each states' militia that is committed to the reinforcement of federal forces if that is needed. The states and the federal government share responsibility for the National Guard.
When the units of the National Guard are not in federal service they are state troops and as such are not prohibited from law enforcement duty under Posse Comitatus. For that reason alone they are suitable forces with which to reinforce the US Border Patrol. There is a provision in law under which the federal government can fund NG operations in crisis support even though these forces have not been called into federal service.
The governors of the border states (except California) have indicated their agreement with the use of their National Guardsmen in the border mission.
I suggest that the resolve of the Government of California to resist federal immigration law should be tested by telling the governor of California that if necessary some or all of the National Guard of California will be called into federal service for this mission. we would then learn how far the Sacramento Nullification clique are willing to go.
"Whenever the President considers that unlawful obstructions, assemblages, or rebellion make it impracticable to enforce the laws of the United States in any state or territory, he may call into Federal service such of the militia of any state. This is another statutory exception to the PCA." wiki on US National Guard
There appears to be ample presidential authority for a federal mobilization of the California National Guard with or without Sacramento's concurrence. pl