Every Tuesday for the last few months, two dozen of the world’s richest people or their advisers hop onto a conference call and plan for war.

The war is against a proposed tax, but not just any tax. It is against a “wealth tax” on California billionaires, one that Silicon Valley’s most affluent see as an existential threat that could usher in a new way of attacking them. So in confidential memos, WhatsApp and Signal chats and conference calls, they are banding together to battle back.

The calls are organized by Building a Better California, a nonprofit started this year to fight Proposition 40, the ballot measure that would impose a one-time 5% tax on billionaires if voters approve it next week. Each week, the nonprofit’s operatives brief donors or their advisers on polling, advertising and opposition research.…