Seoul - South Korea's main opposition party said yesterday it had agreed in principle to join forces with a smaller party to field a joint candidate for the December 18th presidential elections. An official of the National Congress for New Politics party said its president, Mr Kim Dae-jung would become the joint opposition candidate for the elections. Mr Kim Jong-pil, leader of the smaller United Liberal Democrats party, would become the prime minister in the next government if their joint candidacy won the elections, the official said.