The ruling Democratic Party of Japan re-elected Prime Minister Yoshihiko Noda as its leader on Friday.

Mr. Noda, who took office late August 2011, pledged to strengthen solidarity within the party.

About 70 of its lawmakers have already quit the party in protest against a tax hike bill, the restart of idled nuclear reactors and the government’s intention to join talks on a U.S.-backed trade deal — the Trans-Pacific Partnership.

Three other candidates were also running for the party leadership — former internal affairs minister Kazuhiro Haraguchi, former agriculture minister Michihiko Kano and former agriculture minister Hirotaka Akamatsu.

The Democratic Party of Japan won a landslide victory in 2009, ending more than a half-century of almost uninterrupted rule by the Liberal Democratic Party.

More In: International | News