TOKYO — Hundreds of candidates took to the streets Tuesday as the official 12-day campaigning period for Japan’s Oct. 22 lower house election kicked off, with Prime Minister Shinzo Abe’s ruling party facing challenges from regrouped opposition forces. Up for grabs are 465 seats in the more powerful of Japan’s two-chamber parliament. Abe pledged his party’s stable leadership as he sought voters’ support, promising a tougher stance on North Korea, as well as social security reform to relieve burdens on younger generations in Japan’s rapidly aging society. “We should stay unwavering,” Abe said.