When Representative Jamaal Bowman of New York won a Democratic primary in 2020 as an untested middle-school principal, his upset was heralded as evidence of the left’s ascent.

Four years later, Mr. Bowman is now the one fighting for his political life, battling to turn back a primary challenge from George Latimer, the Westchester County executive heavily backed by the American Israel Public Affairs Committee.

The results of Tuesday’s contest in the 16th Congressional District, which covers parts of Westchester County and the Bronx, may test the durability of the Democratic Party’s progressive faction: If Mr. Bowman was to lose, he would be the first member of the House’s left-wing “squad” to be unseated.

With Mr. Bowman trailing in the polls, some of the left’s biggest luminaries have come to his defense, including Representative Alexandria Ocasio-Cortez and Senator Bernie Sanders of Vermont, who joined Mr. Bowman at rallies over the weekend.

But their late-stage support has been countered by a monthlong fusillade of political advertising on behalf of Mr. Latimer. In barely a month, an AIPAC-affiliated super PAC has spent $14.5 million — up to $17,000 an hour — on the race.

Here’s what to know.

Jamaal Bowman was elected to the House during the height of the Covid-19 pandemic in 2020, after his primary upset of Eliot L. Engel, a 16-year incumbent backed by Hillary Clinton, House Speaker Nancy Pelosi and Andrew M. Cuomo, then the governor of New York.