Moon of the liberal Democratic Party garnered 42.6 percent while Ahn, affiliated with the center-left People's Party, recorded 20.9 percent, according to the Realmeter survey conducted from April 27-29 on 1,523 voters.
Moon's reading was 1.8 percent points less than what he received in the previous poll from April 24-26, but he maintained his substantial lead over his runners-up.
Meanwhile, Hong Joon-pyo of the conservative Liberty Korea Party gained 16.7 percent, up 3.7 percent points and just a few percentage points behind Ahn, according to the poll results.
"After reaching a peak of 38.2 percent on April 10, Ahn has been in a downturn losing more ground to Moon and allowing Hong catch up. The difference between Ahn and Hong is within the margin of error," Realmeter said.
According to the survey, Hong was leading the poll among voters in their 60's while overtaking second place among voters in Daegu and the North Gyeongsang Province, the traditional home turf for conservatives.
Among voters who identified as conservative, Hong gathered 46.8 percent while Ahn got 19 percent despite his efforts to appeal to right-wingers.
