By Michele Kambas and George Georgiopoulos ATHENS (Reuters) - An election campaign in Greece ends on Friday with opinion polls giving no clear winner, setting the stage for a cliffhanger vote on Sunday between the political left and right, and the near-certainty the next government will be a coalition. Five opinion polls on Thursday and Friday underlined the tightness of Greece's election campaign, offering different outcomes but all pointing to no outright winner when ballots are cast. All the polls showed the leftist Syriza party of former prime minister Alexis Tsipras and the conservative New Democracy of Vangelis Meimarakis within spitting distance of each other.