The latest poll, conducted by Gallup Korea this week, showed An with 19 percent support, up 9 percentage points from a week earlier, while Moon, a former leader of the main opposition Democratic Party, scored 29 percent, down 3 percentage points.
Although Moon cemented his lead, An is emerging as a possible game changer in the race for the presidency.
Behind An's rise are his ideas of highlighting integration and cooperative governance with opponents.
The Korean Political commentator JS Suh analyses the issue.
