Russia’s Supreme Court threw Yabloko off September’s Duma ballot. The antiwar party has just 4,000 members, but 8,000 people have applied to join in the last two weeks.

Yabloko’s election campaign became Russia’s biggest political story of the summer before it had even properly begun: the Supreme Court removed the party from the ballot, and hundreds of young people came out to support Yabloko. Meanwhile, one of Yabloko’s leaders, Lev Shlosberg, was sentenced to 11 years on trumped-up charges. Amid all that, the party is riding an unprecedented surge of popularity: thousands of people have already submitted applications to join, and Yabloko has picked up hund...