Such an alliance should be done before the deadline for selection of a single liberal candidate this month, Yonhap News Agency reported.
Talks between Democratic United Party nominee Moon Jae-in and popular independent candidate Ahn Cheol-soo can be separate from discussions about which single candidate to select, said Lee Joung-woo, a senior DUP official.
Campaign platforms of both candidates are similar, Lee said, and combining their policies would demonstrate that both candidates are serious about political reform.
Ahn remarked last month that while he didn't object to the parties uniting under a single candidate, talks shouldn't be before Nov. 10 when he plans to announce his platform.
Moon is to announce his campaign pledges Nov. 11.
Ahn is favored over Moon in local polls.
Liberals would lose a three-way race, most nationwide polls indicate. The outcome is more unpredictable if Ahn or Moon withdraws.
Kim Moo-sung, an election strategist for the ruling Saenuri party, charged Ahn and Moon were colluding to steal the election.
Candidates must register with the National Election Commission by Nov. 26.