By Jack Kim

SEOUL (Reuters) - Park Geun-hye, daughter of the founder of modern-day South Korea, has been bumped off the top place in opinion polls of potential candidates for president for the first time in three years.

Park received 40.7 percent of support as a potential presidential candidate, trailing software developer Ahn Cheol-soo with 41.5 percent, a poll of 500 respondents by the Chosun Ilbo newspaper showed on Thursday.

Analysts said the surge by Ahn, a pioneering computer software developer who has never held elected office, reflected deep public distrust of the political establishment and exposed Park's vulnerability to a challenge from the left.

"It's safe to say Park's runway victory is now hardly a certainty," said political analyst Yu Chang-seon.

"What's ahead is Park having to go through fierce competition and we shouldn't rule out Ahn's candidacy."

Park, who stepped into the role of first lady when an assassin's bullet meant for her father Park Chung-hee killed her mother in 1974, had been by far the most popular choice to be the next leader of Asia's fourth largest economy.

South Koreans go to the polls next year to elect a new parliament in April and to pick a new president in December.

Park, who had led the ruling Grand National Party (GNP) previously and favors fiscal conservatism, had led in polls with more than 40 percent support.

But new polls showed she was behind Ahn within the margin of error. Ahn had never been considered a potential candidate for public office before he emerged as a possible candidate for mayor of Seoul.

Ahn, who founded the online security firm Ahnlab, has declared he would not run for the mayoral office in a by-election in October and backed an independent liberal candidate instead, boosting his popularity past previous frontrunners.

He has not spoken publicly on policy but is generally considered a liberal.

Park, 59, has called for tax cuts for businesses to spur investment and jobs, and advocated tailored welfare programs to encourage work for young people, increase the country's lagging birth rate, and improve the quality of life for the elderly.

She has also expressed a willingness to engage North Korea more than the incumbent President Lee Myung-bak, saying positive overtures from Pyongyang must be rewarded with concrete steps to softening the mood of confrontation between the rivals.

Soft-spoken and cautious in demeanor in stark contrast to her autocratic father who was assassinated by his spy chief in 1979, Park has consistently been one of the most popular political figures since election to parliament in 1998.

(Editing by David Chance and Yoko Nishikawa)