A snowballing scandal over government surveillance of civilians is dominating the build-up to South Korea’s parliamentary election on April 11.

Dubbed “Korea’s Watergate” by Seoul’s newspapers, both main parties are accusing each other of illegally monitoring critical businessmen, journalists, activists and unionists. The scandal could weigh heavily on a vote that comes at a defining moment for South Korea, with Seoul running out of time to tackle painful structural reforms that will help it better withstand the rise of Chinese industry.

A state broadcasting union last week published 2,619 files on a leaked memory stick, supposedly compiled by a policeman. The documents suggested a government body that was supposed to investigate corruption among civil servants was actually snooping on civilians who opposed the government.

South Korea’s leftwing opposition, the Democratic United Party, seized the opportunity to attack the conservative administration of president Lee Myung-bak, who already faces accusations of smothering civil liberties in Asia’s fourth-biggest economy.

Han Myeong-sook, chair of the DUP, accused Mr Lee of a “reign of terror” that dragged Korea back to the military dictatorship, which fell in the late 1980s.

“The spectre of the [Korean] Central Intelligence Agency from the authoritarian administration of Park Chung-hee is still haunting Korea,” she said in remarks that target General Park’s daughter, Park Geun-hye, who is the leading conservative candidate for president.

Accusations that Seoul is backsliding towards authoritarianism can have a powerful effect around elections. Mr Lee suffered a heavy defeat in midterm regional elections after he revived memories of military rule by cracking down on people who criticised the government’s account of the sinking of a South Korean warship by North Korea in 2010.

However, the DUP’s attack is backfiring. The government retorted that 80 per cent of the spying cases on the memory stick happened under Roh Moo-hyun, Mr Lee’s leftwing predecessor. The government adds the cases from under Mr Lee’s rule are not new and were examined as part of an inquiry in 2010 that stamped out illegal surveillance by government. Seven government officials charged in that investigation are awaiting verdicts.

Ms Park has tried to distance herself from Mr Roh and Mr Lee by saying: “We have to cast off this kind of wrong, old politics”. She added government spies possibly even kept tabs on her.

It is unclear which party will suffer most in the tussle. Polls differ widely ahead of next week’s election with one major pollster showing the parties neck-and-neck, while another says the conservatives have a 9 percentage point lead.

A conservative victory next week would be a major turn-around because leftwingers had been leading polls over recent months by promising to increase welfare and restrict the dominance of major conglomerates.

However, leftwing politicians have also frustrated voters with inconsistent opportunism, attacking a trade deal with the United States and the construction of a major naval base, two major projects which Mr Roh had supported while in power.

Still, Park Chang-hwan, a professor of politics at Jangan university, predicted younger people would turn to the DUP as the scandal would rekindle underlying resentment about undue interference by government, such as an incident in 2009 when a popular anti-government entertainer lost his slot on state television.

“The administration’s defence that the previous government also conducted illegal surveillance is not adequate,” he said.

Additional reporting by Kang Buseong