South Korea's ruling party celebrated yesterday after sailing through a key by-election test that increased its parliamentary majority, despite lingering public anger over the handling of April's Sewol ferry disaster. President Park Geun-hye's Saenuri Party won 11 of an unprecedented 15 by-elections contested on Wednesday, leaving it with 158 legislators in the 300-seat National Assembly. The results were a blow for the main opposition New Politics Alliance for Democracy (NPAD), whose two co-leaders responded by tendering their resignations. The victory will boost Park's push to restructure South Korea's export-dependent economy through economic stimulus and regulatory reform. The NPAD, which had been hoping to see Saenuri lose its majority, won the remaining four contests, leaving it with 130 seats in the assembly. The result was especially pleasing for Park after June 4 local elections had delivered a split vote between the ruling and opposition groups. As with the local polls, the opposition had sought to make the by-elections a referendum on the Park administration's handling of the Sewol disaster that claimed around 300 lives - most of them high school students. Park's popularity ratings plunged in the wake of the disaster, which was largely blamed on ineffective regulation and corporate greed. The NPAD strategy failed to resonate with voters, and its co-chairmen - Ahn Chul-soo and Kim Han-gil - announced they were stepping down. "I hold myself responsible for the election loss and I am resigning", said Ahn, a former software tycoon who had enlivened the December 2012 presidential election by running as an independent. Ahn had only served as NAPD co-chairman for four months following the merger of his new splinter party with the larger Democratic Party.