The ruling Saenuri Party has seen its approval ratings fall to 33.6 percent, Monday, down from 35.3 percent in the previous week.
The drop in the party's support came amid lingering controversy over a bribery scandal involving deceased Keangnam Enterprises Chairman Sung Woan-jong and a number of figures from the governing camp.
The number is the lowest among previous surveys conducted by Realmeter since the 19th National Assembly was launched in May 2012.
The drop in the party's popularity has been in sync with that of President Park Geun-hye.
Park's ratings have decreased from 41.8 percent in the first week of April to 36.8 percent by the fourth week.
The pollster analyzed that the Sung Woan-jong scandal and subsequent resignation of Prime Minister Lee Wan-koo affected the popularity of the President and the governing party.
On the other hand, the main opposition New Politics Alliance for Democracy (NPAD) received a 30.3-percent rating, up from 28.6 percent the previous week.
The gap in the approval ratings between the rival parties has been narrowed to 3.3 percentage points from 6.7 percentage points.
"A rise in popularity for the largest opposition party seems to have come as liberal voters have unified ahead of the April 29 by-elections," the pollster said.
Sung, who was found dead on April 9 after an apparent suicide, left a list of politicians who allegedly accepted bribes from him. The list mostly includes figures from the ruling party including Prime Minister Lee; South Gyeongsang Province Governor Hong Joon-pyo; Incheon Mayor Yoo Jeong-bok; and Rep. Hong Moon-jong.
The governing party has been wary that the graft scandal will hinder them in the by-elections.
The main opposition party has been attempting to use the scandal as a chance to boost its candidates in the lead up to the elections.
Follow Jun Ji-hye on Twitter @TheKopJihye