The two leading opposition candidates for Seoul mayor agreed Saturday on the terms of a public opinion survey that will determine which of them runs as the single unified candidate in next month's by-election.

Oh Se-hoon of the main opposition People Power Party and Ahn Cheol-soo of the minor conservative People's Party reached the deal after closed-door working-level talks between the two campaigns, both sides said.

The public opinion survey will be conducted on mobile phone users only, not landline users, and ask questions about the candidates' suitability and competitiveness, in a compromise between the two men's wishes, they said.

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 ruling bloc. (Yonhap)