The Israeli political leader whose support could prove key to determining who becomes the next prime minister, is a popular right leaning former member of the Knesset — and was a longtime member of Benjamin Netanyahu’s party.

But Moshe Kahlon, 54, was holding his cards close Tuesday night, saying he spoke with leaders of the two leading parties and would only make his decision after all the votes were counted.

“Moshe Kahlon is the most important person in the political establishment this evening,” Tamar Ish-Shalom, an anchor for Channel 10 News, said in a broadcast on Tuesday night.

Mr. Kahlon may be a career conservative, but he also broke with Mr. Netanyahu in a public spat and started his own centrist party, called Kulanu, in December. According to exit polls cited by the Israel media, Mr. Kahlon’s slate won nine or 10 Knesset seats, enough to give either side in the battle to form a new government a critical edge.