Is there a rising progressive tide in the Democratic Party? Liberals like to claim that there is. But beyond the recent elections of two vocal populists—Senator Elizabeth Warren of Massachusetts and New York Mayor Bill de Blasio—there hasn't been a whole lot of evidence to point to. Indeed, in two of the most hyped challenges to centrist Democratic officeholders—the recent primaries of New York Governor Andrew Cuomo and Chicago Mayor Rahm Emanuel—the left has come up short. And things aren't looking much better for Vermont Senator Bernie Sanders in his left-wing challenge to Hillary Clinton.

On Tuesday, though, progressives scored a victory. The Democratic primary for Philadelphia mayor pitted a crusading left-winger against a charter-school advocate backed by suburban hedge-fund magnates. This time, the left-winger, a former city councilman named Jim Kenney, actually won. Given the city's overwhelmingly Democratic tilt, the primary is likely to decide the election.

Kenney ran on a de Blasio-esque platform of establishing universal pre-K, raising the minimum wage to $15 per hour, and ending stop-and-frisk police tactics. (Kenney, a 57-year-old Irish-American, also epitomizes the classiness and tact for which Philadelphia sports fans are famed: In December, he called New Jersey Governor Chris Christie "fat assed" and "a creep" for sitting in the Dallas Cowboys' box at an Eagles game.) On Monday, de Blasio endorsed Kenney, saying the two shared a "progressive vision." Kenney’s campaign was outspent many times over by supporters of his chief rival, Anthony Hardy Williams, a state senator who was backed by a $7 million super PAC. But on Tuesday night, Kenney took 56 percent of the vote.