South Koreans are voting in a parliamentary election that many predict will hand President Park Geun-hye's conservative party a decisive win, despite frustrations over a sluggish economy.

If Wednesday's 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 amid high unemployment and household debt 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.