The July 30th polls carry extra weight because 15 parliamentary seats are up for grabs, the largest total ever for a by-election.
The stakes are perhaps greatest in the Seoul and Suwon constituencies, which are considered strategically important as barometers of public sentiment.
Candidates of the ruling Saenuri Party have made economic recovery their main platform.
They've been touting their party's connection to President Park Geun-hye and promising reforms.
"There are two choices in this election: elect our candidates, who will bring about economic development for their electoral regions, or their candidates, who are mired in collusive practices and old politics."
The main opposition New Politics Alliance for Democracy has also gone on the offensive, deeming the by-election a chance to prompt change.
"We must provide a warning to the ruling Saenuri Party and the presidential office so that the government brings about changes."
The main opposition party has been hammering on the government's failure to arrest Yoo Byung-eun, the de facto owner of the Sewol-ho ferry, which sunk in April, killing 304 people.
To make them more competitive in races against their conservative rivals, the opposition recently unified campaigns in specific constituencies behind one candidate.
The by-election comes less than two months after local elections in June, in which neither the ruling nor the main opposition party claimed victory.
Ji Myung-kil, Arirang News.