If California declares itself a "sanctuary state," it is saying that it is no longer obeying the laws of the United States. In other words, it has seceded from the nation, just as surely as Southern states seceded from the Union in 1861.
President Trump will have no choice but to treat California as President Lincoln treated the secessionist South in 1861. He will send federal forces into Sacramento to seize the Capitol and take the governor into custody. He must federalize the California National Guard, as Eisenhower did in the South during the school-segregation crisis. He will require the resignation of every mayor, sheriff and police chief who signed on to the sanctuary state and who refused to carry out the duties of their offices. Any who do not resign will be taken into federal custody.
No federal funding will go to California until a new government is installed. Any physical resistance will be met with force. Any other state contemplating sanctuary status must be given an object lesson of what will become of them.
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By the way, once California has been declared to be in a state of secession, its entire congressional delegation, including both senators (both Democrats, by the way), will not be able to vote in Congress. The precedent was set during and after the Civil War. Any other states with delusions of grandeur may want to consider that.