HONG KONG—In the fall of 2016, a millennial pro-democracy candidate became the city’s youngest lawmaker, elected by voters in the most affluent district and unleashing a new political force opposed to Beijing’s creeping control.
Eighteen months on, that candidate, 24-year-old Nathan Law, has been ousted from office for flubbing his swearing-in oath in a perceived snub to Chinese rule. His party, Demosisto, has had its replacement candidate, Agnes Chow, barred from running in special elections Sunday to fill the seat he vacated,...