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California gave rise to Silicon Valley, one of the most powerful and lucrative industries in world history. Now California is poised to rein it back in.

This week’s cover story in The Times Magazine shows how an unlikely band of privacy activists took on Google, Facebook and other companies that mine personal data for profit. The group won a surprising victory in Big Tech’s backyard.

Over two decades, Silicon Valley built a political machine that fended off virtually every effort to regulate how tech companies collect and use data. And when an Oakland real estate developer named Alastair Mactaggart decided to fund a privacy ballot initiative in California, he learned just how powerful that machine was.

Mr. Mactaggart’s initiative proposed giving Californians more rights over their own data, with the power to demand that big companies stop selling and trading the information they collected about browsing habits and location.