On Oct. 12 and 13, members and associate members of the Social Democratic Party cast ballots to elect a new party leader. This year’s election was particularly meaningful because, unlike previous elections for the past 17 years that were uncontested, this year’s saw two men face off: SDP policy chief Mr. Tadatomo Yoshida and Tokyo’s Toshimi Ward Assembly member Mr. Taiga Ishikawa.

With 71.67 percent of the party’s 17,410 members and associate members voting, Mr. Yoshida cruised to victory with 9,986 votes against Mr. Ishikawa’s 2,239, and was elected party chief Monday.

Mr. Yoshida’s mission is clear but difficult. An Upper House member with just three years’ experience, he must rebuild the marginalized party into a viable organization that can once again wield influence in the nation’s policymaking process.

He should remain keenly aware of the fact that the SDP is the only party in Japan that advocates social democracy and belongs to the Socialist International — a worldwide organization of 154 parties and organizations that seek to establish democratic socialism — at a time when many people’s lives are being negatively impacted by market forces.

He needs to work out feasible policy measures that will improve people’s quality of life, and strengthen the party’s organizational power and its ability to inspire people to take part in grass-roots movements to better their lives.

The party’s fortunes have fallen over the years. In 1958, the party’s predecessor, the Japan Socialist Party, held 166 seats in the Lower House. In June 1994, JSP chief Mr. Tomiichi Murayama became prime minister, heading a coalition government composed of the Liberal Democratic Party, the JSP and Sakigake.

The JSP changed its name to the SDP in 1996. At that time the party had 99 Diet members. The SDP took part in a coalition government formed in 2009 under the leadership of the Democratic Party of Japan.

But the party won just one seat in this year’s July 21 Upper House election — leading Ms. Mizuho Fukushima, who had been at the party’s helm since November 2003, to step down to take the responsibility for the poor result. It now has only five Diet members — the minimum number necessary to receive subsidies under the political parties support law.

Mr. Ishikawa, who has made clear his willingness to assist Mr. Yoshida in his efforts to revitalize the party, can be a valuable asset for the SDP. At the age of 28, Mr. Ishikawa publicly came out as gay in 2002, releasing his memoir, titled “Where is My Boyfriend.” He has long been active in the gay rights movement. He could help channel the voices of marginalized people, including irregularly employed workers and members of the gay community, to ensure that they are reflected in local and national politics.

The SDP’s mission to build a more equitable society is becoming all the more important as the gap between the rich and the poor widens under the Abe administration’s economic policy.

The SDP, a strong supporter of Japan’s postwar pacifist policy, must go beyond merely shouting “Protect the pacifist Constitution!” It must work out concrete measures to ensure that Japan’s “defense-only defense” posture is maintained.

To this end, Mr. Yoshida must make strenuous efforts to prepare the SDP for a series of local elections that will be held in June 2015.

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