Exit polls from South Korea's parliamentary election indicate that President Park Geun-hye's conservative party would not regain its majority in the National Assembly.
Such an outcome would jeopardise Ms Park's plans to push ahead with controversial economic reforms, and would blow open next year's presidential race.
Pollsters had predicted that the ruling Saenuri Party would crush a divided opposition for a decisive win and raise its expectations to take the presidency in 2017, after Ms Park's single term expires.
Exit polls by three national television networks unveiled after voting ended at 6pm showed Saenuri managing 118 to 147 seats in the 300-seat assembly.
The networks projected the main opposition Minjoo Party winning 97 to 128 seats, and the People's Party, a new party created mostly by those who defected from Minjoo, taking 31 to 43 seats.
It remained to be seen whether the projections would prove to be accurate, as South Korean television networks have a spotty record of predicting election results with their exit polls.
Official results were expected early Thursday.
There has been disappointment among South Koreans over a sluggish economy, but it seems that criticism of Ms Park's economic policies has taken a back seat to national security issues following North Korea's recent nuclear test and long-range rocket launch.
Hostility between the rival Koreas in election years has often been seen as helping the conservatives by allowing them to highlight their hard-line approach against the North.
Liberals have traditionally backed rapprochement policies with the North.
Officials from the main opposition Minjoo Party had expressed concerns that Saenuri could achieve something close to a two-thirds supermajority in the National Assembly.
Official figures show household debt at new highs and the unemployment rate for people under 30 approaching levels not seen since the late 1990s, when millions lost their jobs during a crippling financial crisis.
In a survey of 1,000 adults unveiled last week by Gallup Korea, the percentage of respondents saying Ms Park was doing a good job as president rose by 5 percentage points from the previous week to 43%.
The survey showed that Ms Park's supporters highly rated her diplomatic policies and stern measures against North Korea, including economic sanctions and the shutdown of a factory park in the North that had been jointly run by the rivals.
Since losing its second consecutive presidential election in 2012, the Minjoo Party has struggled with factional infighting and lawmaker defections, and saw its seats decline from 127 to 102 in the current assembly.
In this year's general election, the Minjoo has been forced to compete for mainstream liberal votes with the People's Party led by its former co-chairman, Ahn Cheol-soo, who is seen as a potential candidate for the 2017 presidential election.
Ahn's People's Party has focused its campaigning efforts in the south-west Jeolla region, which has traditionally been the core liberal support base.
South Korea's electorate is deeply divided along generational and ideological lines, and also by fierce regional loyalties. Voters in the south-east Gyeongsang regions have for decades overwhelmingly voted for conservatives in parliamentary and presidential elections.
The National Election Commission estimated that 58% of the country's 42 million voters participated in Wednesday's election, a higher turnout than four years ago, when 54.2% of the electorate voted.