Prime Minister Shinzo Abe's crushing win in national polls last month -- his second in two years -- was believed by some commentators to be largely due to the absence of a credible alternative.
The Democratic Party of Japan, which governed for three years until December 2012, won just 72 seats in the 475-seat lower house. Abe's Liberal Democratic Party has 290.
The party's tally -- barely up on their drubbing in 2012 -- did not include leader Banri Kaieda, who lost his seat.
Three heavyweights lined up Wednesday to present their cases for taking his place, all trying to put the punch-drunk party back on an even keel.
The presumptive frontrunner is one-time leader Katsuya Okada, a 61-year-old Harvard-trained former deputy prime minister, widely known for his policy knowledge and strict self-discipline, including refusal to accept all gifts, even Valentine's Day chocolate.
He appeared at Wednesday's press conference wearing an eye-patch, after reportedly undergoing eye surgery.
He faces challenges from the telegenic Goshi Hosono, 43, a media-savvy former minister in charge of handling the Fukushima crisis and Akira Nagatsuma, 54, known for his desire to refashion the public pension system.
Whoever wins will have his work cut out rebuilding public trust in the nominally centre-left party, whose three years in power to December 2012 were characterised by power struggles, policy flip flops and diplomatic mis-steps.
The party was also criticised for its handling of the Fukushima crisis in the aftermath of the killer 2011 tsunami.
Commentators warn that a directionless opposition party is no good for Japan's polity, and allows Abe almost unfettered rein.
They point to the record low 52.66 percent turnout in the general election as proof of voters' disillusionment with a system of governance often criticised for pandering to vested interests.