In his final months, Kan mounted a fierce campaign against the country's nuclear power industry, shutting down the vulnerable Hamaoka Plant in Shizuoka. While his policy to phase out nuclear energy resonated with voters, it didn't improve his approval ratings, which fell below 20 percent.
Sone says Kan was hurt by political infighting and a divided government, but he didn't help his cause with his inability to communicate with the public.
"The voters didn't trust Kan as a leader," he said. "Kan's ideas were proposed on a whim. They weren't very thought out. He failed to explain the larger picture."
As many as seven DPJ lawmakers are expected to vie for party leader. Former Foreign Minister Seiji Maehara, and current Finance Minister Yoshihiko Noda are considered frontrunners.
Kan has not backed either candidate, but called for unity, as the DPJ votes for the next prime minister.
"I hope to see this party become one, where everyone from the young to the veterans can discuss policy actively and freely, then cooperate and act as one," he said.