Although Mitsotakis has been steadily ahead in opinion polls, a newly introduced electoral system of proportional representation makes it unlikely that whoever wins the election will be able to garner enough seats in Greece’s 300-member parliament to form a government without seeking coalition partners. The two main contenders in Sunday’s vote are conservative Prime Minister Kyriakos Mitsotakis, 55, a Harvard-educated former banking executive, and 48-year-old Alexis Tsipras, who heads the left-wing Syriza party and served as prime minister during some of the financial crisis’ most turbulent years. ATHENS, Greece — Polls have opened in Greece’s parliamentary election, the first since...