Nullification as a constitutional theory rests on the belief that the United States is an artifact of an agreement among the states as to a form of government agreeable to them collectively and individually and that this agreement does not oblige the states to accept laws enacted by the US Congress that the states find unconstitutional.
The ability of the states to secede from the Union is thought to rest on the language of the Declaration of Independence which states the right of a people to determine the form of their government. In addition to that, the US Constitution contains no language that requires states to accept that the union is indissoluble. It may be argued that this issue was settle 150 years ago, but, in fact it was not. Force majeure decided that test of the right of the states to secede, and that settled nothing.
California is now embarked on a course of nullification of federal law and regulations. Becerra, the new AG of the state seems to see himself as a 21st Century Joaquin Murietta, the mythic 19th Century leader of Hispanic resistance to US government authority.
The state is refusing to cooperate with federal authorities on a variety of immigration issues. The state legislature has now passed a law that declares the state to be a sanctuary for people illegally resident in the United States. Governor Brown is expected to sign this into law. To my mind that signature will put California in a state of rebellion against the federal government.
From this to secession from the Union is a plausible step if the nullification of federal law is successful. pl