Representative Alexandria Ocasio-Cortez of New York has declared the race her most pressing electoral priority. Senator Bernie Sanders of Vermont is slated to join her for a pre-election rally in the Bronx. And a powerful pro-Israel lobby working for the other side has already shattered spending records.

The House primary between Representative Jamaal Bowman, a left-wing Democratic darling, and George Latimer, a more moderate challenger, was always expected to be a marquee showcase of their party’s divisions over the Israel-Hamas war.

But with early voting underway, the June 25 contest in New York has exploded into a proxy war for the Democratic Party’s future, tearing open old wounds over race, class and ideology in the heart of a presidential election year.

On one side, nearly the entire local Democratic establishment, including Mr. Sanders’s 2016 rival, Hillary Clinton, has lined up behind Mr. Latimer, the Westchester County executive. So has the American Israel Public Affairs Committee, a pro-Israel lobby that has already spent $12 million — more than any other race in its history — to try to defeat Mr. Bowman over his criticism of Israel.

On the other side, Ms. Ocasio-Cortez, the other members of the House’s left-wing “squad” and their progressive coalition are undertaking a desperate rescue mission, draining bank accounts and leveling increasingly personal attacks as polls indicate that Mr. Latimer has opened a commanding lead.

Mr. Bowman’s allies fear his loss would not merely deprive the movement of one of its most charismatic spokesmen but might also embolden forces trying to pull the party back toward the center on issues like climate change, immigration policy and the war in Gaza.