The Japanese Communist Party and the Social Democratic Party, two small but enduring opposition groups in the Diet, lost seats in Sunday's general election as several "third-force" groups emerged to challenge the status quo.

The JCP, headed by Kazuo Shii, slid from nine seats to eight, while the SDP, led by Mizuho Fukushima, was reduced to just two seats from five.

Both parties pledged to scrap the Democratic Party of Japan-led government's sales tax hike, halt all nuclear plants and oppose Japan's participation in the U.S.-led Trans-Pacific Partnership free-trade talks.

These were some of the same planks set down by the third-force parties, such as Nippon Ishin no Kai (Japan Restoration Party), led by former Tokyo Gov. Shintaro Ishihara, and Nippon Mirai no To, (Tomorrow Party of Japan), led by Shiga Gov. Yukiko Kada.

Fukushima said the SDP had "found it difficult to differentiate (itself) from other parties" on those key issues.

Shii told reporters the outcome was not because "the LDP received a boost but (because) the DPJ faced strong headwinds."The JCP won 20 seats in the 2000 general election but retained only nine in the subsequent 2003, 2005, and 2009 races.