In a Hankook Research poll conducted Monday and Tuesday, Moon (40.4 percent) beat Ahn (26.4 percent) by a margin of 14 percentage points.

Ahn's decline was widely attributed to his performance in a series of presidential debates, which some said was unprofessional, and negative allegations involving his campaign team and his wife.

In a Metrix survey conducted Sunday and Monday, Moon garnered 40.3 percent, 10.7 points ahead of Ahn's 29.6 percent. It followed a Gallup Korea poll last week in which Moon rose 1 point to 41 percent and Ahn dropped 7 points to 30 percent.

Moon's campaign hailed the development, saying it will shift its focus from the competition to state affairs planning.

"We now see one strong (candidate), one moderate and one weak," Kim Kyung-soo, a spokesman of the team, said in a phone call with Yonhap, apparently referring to Hong Joon-pyo of the conservative Liberty Korea Party, who has ranked a distant third.

Officials at Ahn's camp, however, interpreted the trend as a temporary adjustment before he rebounds in the final stretch of the campaign.

The choice of swing voters will be important in the run-up to the May 9 election, observers say. They now account for 21.3 percent of the electorate, up from 14.5 percent two weeks earlier, according to a Kantar Public survey published Sunday. In the same period, the proportion of voters willing to shift their allegiance also increased from 30.8 percent to 34 percent.

Apparently desperate to stop Moon, the splinter conservative Bareun Party on Tuesday proposed a three-way alliance involving Ahn, Hong and its own nominee Yoo Seong-min. But the offer was immediately dismissed by all of them. (Yonhap)