Silicon Valley Rich Fear Union That Could Cost Them Billions (1)

The California wealth-tax plan that’s drawn ire from Silicon Valley billionaires and Governor Gavin Newsom has a familiar force behind it: a union adept at turning ballot initiatives into a potent political tool.

The SEIU United Healthcare Workers West is gathering signatures for a ballot measure that would impose a one-time 5% tax on wealth above $1 billion. The union, which represents about 120,000 workers, says the plan will force California’s richest residents to bolster the state’s health-care system.

Gavin Newsom

Photographer: Mauro Pimentel/AFP/Getty Images

The campaign — which has sparked outcry over the prospect of top taxpayers leaving — is a vivid example of ...