BEIJING—Li Xiaopeng once idolized the West. While a student, he broke through China’s internet firewall to read news from abroad, revered the U.S. Constitution and saw the authoritarian Chinese government as destined to fade away.

Now the 34-year-old urban consultant, who studied at both Cambridge and Harvard, thinks it’s China that is ascendant and the U.S. that is terminally weakened by income inequality, divided government and a polarized society. He says so volubly to his more than 80,000 followers on social media.

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