A first-in-the-nation proposal in California to tax billionaires could test voters' appetite for levying a wealth tax on the ultra-rich. Vermont Sen. Bernie Sanders is headlining events to urge Californians to pass the measure. But it faces significant political headwinds. Billionaires are spending millions to fight the tax. And Democratic Gov. Gavin Newsom and other major players in the state capital are warning that it could have unintended consequences. They are worried that wealthy residents are leaving the state and taking what they would have contributed in income tax revenue with them.