TAIPEI, April 14 -- The upcoming chairmanship election for Taiwan's main opposition Democratic Progressive Party (DPP) is likely to become a one-woman show, with former chairwoman Tsai Ing-wen being the sole candidate for now.

Her other two rivals, incumbent chairman Su Tseng-chang and DPP heavyweight Hsieh Chang-ting, declared that they will drop out of the May election on Monday.

Scheduled for May 25, the DPP chairman election opened candidacy registration on Monday and will last until Friday.

The withdrawal of Su and Hsieh came amid mounting criticism inside the DPP about the party's failure to lead a student protest against a cross-Strait service pact, which ended on Thursday last week.

There have also been stronger calls for the party to break its factions and for party elders to hand power to the younger generation of leaders.

Su, 66, said in a statement on Monday morning that he would not stand for re-election to avoid infighting and damage to the party. Hsieh, 67, later also announced the decision, saying he had hoped to prevent worsening of factional confrontation.

Following Su's and Hsieh's announcements, Tsai, 57, said in a statement that the DPP should reflect on its role against the backdrop of rising social forces, referring to the student protest. She also called for party reforms and power transition between generations.

Tsai acted as DPP party chief between 2008 and 2012 before stepping down after losing to Ma Ying-jeou in the Taiwan leadership election in 2012.