The night before Tuesday’s Democratic primary in New York City, Zohran Mamdani and Brad Lander appeared on “The Late Show With Stephen Colbert,” bringing their progressive bromance to a national viewership. Ostensibly running against each other to be New York’s next mayor, their alliance showcased what parliamentary-style coalition politics could look like in the age of so much vitriol and polarization.

Not long before early voting began, the candidates cross-endorsed each other in the name of an ideological victory and the defeat of the better-known, better-funded front-runner. “We both agree that corrupt, abusive Andrew Cuomo should not be allowed anywhere near City Hall,” Mr. Lander, the city’s comptroller, said on the show, as the studio audience cheered. And now it looks as if he won’t.

Against the predictions of nearly all polling, Mr. Mamdani is on track for a decisive win — and not after the tallying of several rounds of ranked-choice voting, assumed to be the only route to defeating an opponent with such an imposing advantage. Mr. Mamdani leads in the first round of counting by 7 percentage points, a margin significant enough that Mr. Cuomo quickly conceded on Tuesday night.

The result could reasonably lead to the assumption that a still-novel method of ballot casting in New York had little to do with the outcome. But in fact, ranked-choice voting, now in place in at least 60 jurisdictions around the country, shaped the competition from the beginning.

It rewards a campaign style that played to Mr. Mamdani’s strengths: ever-present, on-the-street, nonstop voter engagement. Mr. Mamdani was doing everything — even jumping into the freezing cold ocean to call attention to his proposal for a rent freeze — and his many thousands of campaign volunteers were everywhere.

All of this stood in sharp contrast to Mr. Cuomo’s I’ve-got-this-locked-up strategy, one that relied heavily on big-money TV advertising, little noticed by voters under 70, and the conviction that there was no one well-known or formidable or experienced enough to beat him.