Ahn, 55, has taken a tougher stance on national security than the liberal frontrunner for president, Moon Jae-in, who won his Democratic Party's nomination on Monday, saying there should be no mistake about the dangers presented by North Korea.
Although a multi-cornered fight is expected, pundits say it will most likely come down to liberal candidate Mr Moon and former software tycoon Ahn Cheol Soo, whose candidacy for the centrist opposition People's Party is due to be revealed today.
Moon Jae-in celebrates after winning the nomination as a presidential candidate of the Minjoo Party, during a national convention, in Seoul, South Korea, April 3, 2017.
Ahn was a top contender for the 2012 presidential election but withdrew his candidacy and endorsed Moon in a bid to unite non-conservative voters against eventual victor Park. The progressive Justice Party chose Rep. Sim Sang-jeong as its nominee in February. Kim Chong-in, a former interim leader of the Democratic Party, is said to declare his presidential bid soon.
The crowded election field is a rarity for South Korean politics that has usually seen the candidates of two or three major parties vying for the country's top elected office in the past elections.
Mr Ahn's approval rating has risen sharply in the past few weeks - to 19 per cent in the latest Gallop Korea poll, nearly double the figure a week ago, as voters turned to him to foil a likely Moon presidency.
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Apparently mindful of Ahn's growing challenge, Moon has been ratcheting up his offensive against Ahn.
In remarks late last month, People's Party chief Park Jie-won expressed confidence in Ahn's chances against Moon.
He initially struggled in the presidential race, triggered by the impeachment of Park, but has shot to second place in recent opinion polls, with 19 percent support in a poll released on Friday.
Mr Moon, while hugely popular with younger liberals, risks alienating older conservatives with his pro-North policies. But the two remain poles apart over the terms of their merger.
The election comes seven months earlier than scheduled after Park was dismissed over a corruption scandal. As a prerequisite for the merger with the largest conservative party, it demanded the removal of party members with ties to the disgraced former leader.
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