Moshe Feiglin wants Israelis to be able to smoke a joint without legal consequences. He also wants Israel to annex the entire West Bank and strip non-Jews of their voting rights unless they pledge loyalty to the state.

Fusing right-wing ultranationalism with once-fringy libertarianism, Mr. Feiglin is shaping up as an unlikely kingmaker who could crown either Prime Minister Benjamin Netanyahu or his main opponent, former Gen. Benny Gantz, in a tightly contested national election April 9.

Polls...