Lawmakers in Japan's struggling main opposition party pledged Saturday to stop the ruling Liberal Democratic Party, headed by Prime Minister Shinzo Abe, from winning a critical election they see as the last chance to preserve the postwar pacifist Constitution.

"This summer's Upper House election is of paramount importance to Japan," Katsuya Okada, president of Democratic Party of Japan, told fellow lawmakers from across the nation at the party's annual convention.

A sense of urgency filled the air at this year's gathering, because Japan, in Okada's words, "is at a crossroads" in defending the supreme code against the right-leaning plans of Abe.

Letting the ruling coalition win a two-thirds majority in the 242-seat Upper House would give Abe a huge chance to amend the Constitution — his long-held goal and one that Okada says threatens Japan's pacifist philosophy and basic human rights.

"We must not lose this fight. We must not let the Abe administration have its way. We need to win back power," Okada said.

The DPJ, however, is likely to face an uphill battle.

The party suffers from lackluster public support stemming from its first — and only — stint in power from 2009 to 2012, which was fraught with broken promises, hazardous foreign diplomacy and an unprecedented triple calamity.

When public anger heated up as the ruling LDP-led coalition bulldozed controversial security bills last year, the DPJ tapped anti-Abe sentiment to oppose them but didn't make any gains.

In a dig at Abe's much-hyped goal to "create a society where all 100 million people can play an active role," Okada said the government's conservative policies appear contradictory.

"How can a nation that easily abandons one person make all 100 million people happy?" he asked. "We, on the other hand, aim to create a coexistent society where no one is marginalized."