Wrapping up a two-day convention in Tokyo, the Social Democratic Party vowed to fight Prime Minister Shinzo Abe's drive to amend the war-renouncing Constitution.

The SDP on Sunday adopted an action plan calling for "bold" cooperation among opposition forces to fight the ruling bloc in this summer's Upper House election.

On Friday, five opposition parties — including the Democratic Party of Japan, the Japanese Communist Party and the smaller SDP — submitted to the Diet two bills that would scrap the contentious security laws that were enacted last year.

Senior executives of the four other parties attended the first day of the SDP's convention Saturday as guests, during which they urged unity.

Under the security legislation that is to take effect March 29, Japan will be allowed to exercise the right to collective self-defense. The legislation will also expand the range of missions the Self-Defense Forces can be sent on overseas.

Meanwhile, in early February Abe called for public debate about amending the Constitution. He has said this will be a central issue in the campaign for the Upper House election.

The SDP action plan expresses alarm about an existential showdown the venerable party may face in this election. It currently has two lawmakers in the Lower House and three in the Upper House — of whom, the seats of SDP leader Tadatomo Yoshida and former leader Mizuho Fukushima are up for grabs.

The SDP began life in 1955 as the Japan Socialist Party and at one time was the largest opposition force.