By Chung Min-uck

The main opposition Democratic United Party (DUP) called for the reinvestigation of an illegal surveillance case allegedly involving Cheong Wa Dae officials Tuesday with less than a month left before the April 11 National Assembly elections.

"The DUP will define the case as the Lee Myung-bak administration's Watergate scandal ordered by Cheong Wa Dae, conducted by the Prime Minister's Office and covered up by prosecutors," said Rep. Kim Jin-pyo, floor leader of the DUP, during a policy meeting, Tuesday.

"A comprehensive reinvestigation by the prosecutors is needed."

On Monday, Jang Jin-su, a former senior official at the Prime Minister's Office (PMO), revealed a recorded conversation between him and Choi Jong-seok, a former staff member at Cheong Wa Dae, which includes evidence that the presidential office tried to cover up the case by bribing Jang.

The scandal initially erupted in 2010 when Kim Jong-ik, a former CEO of KB Hanmaum, a subsidiary of KB Financial Group, claimed government officials monitored him after he posted video clips critical of the President on his blog in 2008.

A prosecution investigation into the case ended the same year after indicting seven officials from the PMO.

No officials from the presidential office were charged over the case despite speculation that it was behind the move.

Jang earlier claimed he was ordered to destroy evidence by Choi while Cheong Wa Dae denied its involvement.

The new revelation is expected to deal a severe blow to the Lee administration and ruling Saenuri Party as the opposition parties seek to shed new light on the case.