If the vote allows the ruling Saenuri Party to comfortably regain its majority over a divided opposition, as pollsters predict, it raises expectations the party will take the presidency in 2017, after Park's single term expires.
Criticism of 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.
Official figures show household debt is at new highs and the unemployment rate for people under 30 is approaching levels not seen since the late 1990s, when millions lost their jobs during a crippling financial crisis.
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In a survey of 1,000 adults unveiled last week by Gallup Korea, the percentage of respondents saying Park was doing a good job as president rose by 5 percentage points from the previous week to 43 per cent.
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 a new party created mostly by those who left Minjoo, including its former co-chairman, Ahn Cheol-soo, who is seen as a potential candidate for the 2017 presidential election.
South Korea's electorate is deeply divided along generational and ideological lines and also by fierce regional loyalties. Voters in the southeast Gyeongsang regions have for decades overwhelmingly voted for conservatives in parliamentary and presidential elections.