Former New York governor Andrew M. Cuomo speaks during the New York City Mayoral Candidates Forum at Medgar Evers College on April 23. (Frank Franklin II/AP)

At first glance, it’s stunning that former New York governor Andrew M. Cuomo and state Rep. Zohran Mamdani are so far ahead of their competitors in the Democratic primary for mayor of New York. After all, the field of candidates, who will debate one another for the first time Wednesday night, includes numerous contenders with more traditional résumés — they aren’t 33 years old like Mamdani or had a government report conclude they sexually harassed 11 women like Cuomo.

But if you follow Democratic politics closely, the ascendance of Cuomo and Mamdani is less surprising. In primaries across the country over the past decade, a bloc of

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