After months of public deliberation and prodding from donors aligned with Israel, George Latimer, the Westchester County executive, said on Wednesday that he would mount a Democratic primary challenge against Representative Jamaal Bowman of New York.

The decision set the stage for a potentially explosive contest next year that promises to test not only the growing Democratic divide over the war in the Middle East but the durability of the party’s progressive wing.

In an interview, Mr. Latimer drew sharp contrasts between himself and Mr. Bowman, one of left’s most vocal critics of Israel. He dismissed the incumbent’s calls for a cease-fire as premature and called a recent protest outside the White House, where the congressman accused Israel of committing genocide in Gaza, a political stunt.

“It’s about results, not rhetoric,” said Mr. Latimer, who has deep ties to the Democratic establishment. “So much of politics has turned into that sort of showmanship — how you look in front of the cameras.”

He was expected to officially begin his campaign with a video announcement later on Wednesday, just days after returning from a wartime visit to the region.

The nascent contest echoes primary fights breaking out from Pittsburgh to Detroit since Hamas’s deadly Oct. 7 attack, as pro-Israel Democrats try to oust members of the House “Squad” pushing for a cease-fire. Like the other challengers, Mr. Latimer is expected to benefit from millions of dollars in outside spending by the American Israel Public Affairs Committee, or AIPAC, and other special interest groups.