On Sunday, Istanbul’s mayoral election — a rerun of the March 31 contest that President Recep Tayyip Erdoğan’s party had annulled — ended with a landslide victory for the opposition candidate, Ekrem Imamoğlu. According to the most recent numbers, the vote margin between Imamoğlu and the regime candidate, Justice and Development Party (AKP) member and former prime minister Binali Yıldırım, increased from about 13,700 votes to an astonishing 806,426, while the spread grew from less than half a percentage point in March (48.8 percent to 48.55 percent) to nearly 10 percentage points on Sunday (54.21 percent to 44.99 percent).

This was nothing less than a crushing defeat for the regime. The last time Turkey experienced such a big swing in voting behavior — and indeed, in the popular mood on the streets — was the parliamentary elections of June 7, 2015, which delivered the AKP its first major electoral defeat in party history.

The result is all the more stunning because Erdoğan himself made clear the goal of the rerun was to prevent a June 7-esque disaster. So, what happened?