Two leading opposition candidates for Seoul mayor will conduct a public opinion poll early this week to determine who should run as the single unified candidate in next month's by-election, their campaigns said Sunday.

The campaigns for Oh Se-hoon of the main opposition People Power Party and Ahn Cheol-soo of the minor opposition People's Party said they agreed during working-level talks to hold the survey from Monday to Tuesday and announce the winner as soon as the results of the survey are out.

The single candidate is expected to be declared on Wednesday, at the latest, one day before the start of the official campaigning period.

The two sides earlier agreed to have two pollsters each conduct a survey on 1,600 mobile phone users, not landline users, and ask questions about the candidates' suitability and competitiveness, in a compromise between the two men's wishes.

Landline users are traditionally deemed favorable to more conservative candidates.

The April 7 election will be a bellwether with only a year left of the Moon Jae-in administration's single five-year term.

Oh, a former Seoul mayor, and Ahn, a former software mogul-turned-politician, have been under mounting pressure to unify their campaigns to improve the conservative bloc's chances against Park Young-sun, the single candidate of the liberal bloc. (Yonhap)