After dominating Turkish politics for more than a decade, few doubt Erdogan will beat his main rival Ekmeleddin Ihsanoglu, a diplomat with little profile in domestic politics, or Selahattin Demirtas, a young Kurdish hopeful. But Erdogan’s opponents say it has been an unfair contest, a charge the prime minister dismisses. An Erdogan victory would concentrate more power in the hands of a man who has divided Turkish society along secular-religious lines and worried Turkey’s western allies. While his rivals have funded their rallies mainly from donations, Erdogan has turned his public appearances, some of them state-financed, into a show of...