How a proposed tax on California billionaires is dividing Democrats ahead of the midterms

As national Democrats search for a unifying theme ahead of the fall’s midterm elections, a California proposal to levy a hefty tax on billionaires is turning some of the party’s leading figures into adversaries just when Democrats can least afford division from within.

Vermont Sen. Bernie Sanders traveled to Los Angeles on Wednesday to campaign for the tax proposal, which has Silicon Valley in an uproar, with tech titans threatening to leave the state. Democratic Gov. Gavin Newsom is among its outspoken opponents...