ERBIL, Iraq—Iraqi voters appeared to hand a political breakthrough in national elections to populist cleric Moqtada al-Sadr, forcing the U.S. and Iran to confront the prospect that a frequent critic of both may take the lead in selecting Iraq’s next premier.
In an election seen as a contest between the U.S. and Iran for influence in Iraq, Mr. Sadr’s strong showing in early, incomplete results scrambled the political landscape. Prime Minister Haider al-Abadi, who had implicit American support, and Iran-aligned candidates lagged...