ATHENS, Greece — Greeks were voting Sunday in a referendum on their nation’s financial future, with opinion polls showing that residents were evenly split on whether to accept creditors’ demands for more austerity in return for rescue loans or defiantly reject the deal. Greek Prime Minister Alexis Tsipras was gambling the future of his 5-month-old left-wing government on the hastily called poll — insisting that a “no” vote would strengthen his hand to negotiate a better deal with creditors while a “yes” result would mean capitulating to their harsh demands. The opposition accuses Tsipras of jeopardizing the country’s membership in the 19-nation club that uses the euro and says a “yes” vote is about keeping the common currency. While voters headed to polling stations, large lines once again formed at ATM machines.