How long would Southern California last without the water from the Colorado River, which flows from red states like Utah and Wyoming? California is in the national power grid, and due to its environmental regulations, must import power from neighboring states. Agriculture is an important to the state's economy. If cut off from its usual markets, it will have to export its produce. Mexico, plus reviving fruit, vegetable, and tree nut production in the Midwest and South will serve as a good substitute.
Without assistance from foreign nations, notably China, including financial assistance, a West Coast republic would be as much a basket case as, say, an independent Scotland would be.