MANILA - (UPDATE) Vice President Leni Robredo on Tuesday said a "united opposition" -- composed of the Liberal Party, civil society groups, and possibly ousted Chief Justice Maria Lourdes Sereno -- will release their senatorial line-up for the 2019 mid-term polls in September.

Sereno is among the candidates being eyed to run under the opposition, Robredo told reporters.

"Nagkausap kami for the first time ni CJ Sereno. Sinabi ko sa kaniya na may nagno-nominate [sa kaniya]. I think kailangan niya ng panahon para pag-isipan, para mag-discern," Robredo said.

(Sereno and I talked for the first time. I told her that someone is nominating her. I think she needs time to think and discern.)

• Sereno open to running for public office
• Robredo: Sereno a 'big addition' to LP's Senate slate

Liberal Party President and Senator Francis Pangilinan said that the opposition coalition being formed by Robredo "revolves around her compassionate, empowering, and responsible leadership."

"Her declaration to lead this coalition only formalizes what has been her role since her election into office two years ago: a stabilizing balance to those in power and their mistaken policies and their abuse of the people’s trust that put them in power," Pangilinan said in a statement.

Other possible candidates under the slate will be selected by a committee composed of coalition members, and will be subject to Robredo's approval.

The group has yet to decide if they will field a full slate for the 12 senatorial vacancies, Robredo said.

The Liberal Party earlier said it was convincing actress Agot Isidro, a vocal critic of President Rodrigo Duterte, to be a part of the opposition's senatorial line-up for the midterm polls.

• Liberal Party eyes Agot Isidro in 2019 Senate slate