By Jack Kim and Ju-min Park

SEOUL (Reuters) - South Korea's liberal opposition, bolstered by the under-40s and power of social media, could spring a surprise win in this week's parliamentary elections despite opinion polls that show it tied with the ruling conservatives.

Experts say traditional pollsters base their projections on owners of fixed telephone lines, whereas people in their 20s and 30s, who form 37 percent of the voting population in the world's most wired country, rarely use them.

The young, more likely to carry a Samsung Galaxy or Apple iPhone in their pockets, are mostly liberal and their views are expressed and spread online, often by their smartphones.

"Views expressed in cyberspace are about 20 percent favorable to us and 80 percent against," said Lee Jun-seok, a 27-year old Harvard-educated computer expert brought in to help revamp the ruling conservative Saenuri Party's online presence.

"It's almost like as soon as you say something for our party, you come under attack."

The five most popular politicians on Twitter are all left-wingers. The top conservative is presidential contender Park Geun-hye who ranks eighth with about 180,000 followers, according to Koreantweeters.com, a website on Twitter power.

On the other hand, a traditional Realmeter poll taken between March 26-30 showed 39.8 percent support for the ruling conservatives, 30.5 percent support for the main opposition Democrat United Party and 8.1 percent for its coalition partner, the United Progressive Party.

At the end of last year, traditional polls had the conservatives trailing the opposition, but now suggest the ruling party has made a comeback. Experts said they were flawed.

"Random digit dialing systems based on fixed lines rule out young people, workers who come home late and households that don't have landlines," said Yoon Hee-woong from the Korea Society Opinion Institute, a research organization.

South Korea has the world's second largest blogging community after China. Twitter use here is twice the world average, according to a Singapore Management University study. And they have enormous leverage in elections because the government has now lifted a ban on campaigning in social media.

"On Twitter, we are like birds talking to each other. That's something that can't be controlled," said Kim Mi-wha, 47, a television comedian with almost 290,000 Twitter followers who is part of a band of celebrity super-tweeters embracing liberal causes.

Super-tweeters like Kim have already helped elect an independent activist as mayor of Seoul, ending conservative control over the capital last year.

Although polls had put the eventual winner, Park Won-soon, ahead in the mayoral race, his victory was much more decisive than indicated.

COUNTERWEIGHT

Social media has also acted as a counterweight to the mainstream media, which is largely controlled by the huge conglomerates that dominate the world's 13th largest economy.

It could prompt young voters who are concerned with issues like growing income inequality to swing behind the Democrat United Party that is opposed to a free trade agreement with the United States and wants restrictions on big businesses.

"I don't know about what kind of tricks politicians are going to play with Twitter but the enormous young population will take part in the vote anyway," Kim, the comedian, said.

Rattled by the opposition's success in mobilizing the youth vote in the Seoul mayoral campaign, the Saenuri Party has responded by boosting its social media credentials, but admits it faces an uphill battle.

Lee, the ruling party expert, says one of the reasons Twitter works so well here is that its 140 character bursts offer a perfect fit for the Korean language that can pack a lot more into its space than English. "In English, basically all you can do is to tell someone to check out a YouTube link. In Korean, you can not only describe what the clip is about but how you feel about it," said Lee.

The Twitter boom has also triggered some distinctly old-school money politics with charges that members of parliament are paying for followers to boost their popularity.

Local newspaper Chosun Ilbo reported that accounts with 20,000 followers were changing hands for 2 million Korean won (about $1,770).

"A lawmaker's office offered me bonuses if I gathered more than 10,000 followers," said a woman who spoke on condition of anonymity and used to manage a lawmaker's Twitter page.

INFLUENCE WILL ONLY GROW

South Korea's parliamentary elections are essentially a dry run for the powerful presidency. That vote in December will be the key test of whether the Twitter-using liberals can turn their lock on cyberspace into hard political power.

Yu Chang-ju, who helped manage Park Won-soon's election win as Seoul mayor and now advises him on new media strategy, says the number of Twitter users will double by the time of the presidential election in December to about 10 million, or about one-fifth of the population.

"Twitter is a tool for many moderate Koreans to vent anger against the rich, the current administration and the ruling party," said Huh Chang-deog, a sociology professor at Yeungnam University.

"This year's elections are going to be the SNS (social networking sites) users' group versus non-SNS group," he said.

Kim, the comedian, says Twitter has the power to make or break politicians' reputations in seconds.

"Politicians will become more scared of SNS and they will be careful about what they say and do… I am hoping for a big social change through Twitter here like the Jasmine Revolution," she said, referencing the protests in Tunisia that helped trigger the "Arab Spring" uprisings last year. ($1 = 1129.5000 Korean won)

(Editing by David Chance and Raju Gopalakrishnan)