Prime Minister Yoshihiko Noda is expected to defeat the three contenders he's facing off against in his ruling party's leadership election later this month — even though he may not be Japan's leader for much longer.

Noda and three others from the Democratic Party of Japan — none of them high-profile politicians — announced Monday that they will run in the Sept. 21 poll. While Noda is expected to win that contest, his days as prime minister may be numbered due to widespread voter dismay over a perceived lack of leadership from his party.

Noda's approval rating has fallen below 30 percent after his government pushed through plans to double the sales tax to meet rising social security costs and restarted two nuclear reactors after all of them were shut down following last year's nuclear crisis in Fukushima. Surveys show the Japanese public overwhelmingly supports a complete phase-out of nuclear energy.

Despite those dim numbers, Noda, who has been in office a year, has suggested he will call general elections soon. Media reports speculate they could be held between November and January.

Polls show that more voters prefer the main opposition Liberal Democratic Party, or LDP, which has ruled Japan for most of the post-WWII era.

Noda is Japan's sixth prime minister in six years — a dismal rate of leadership turnover for a country that is battling enormous problems, including a stagnant economy, aging population and growing national debt. It is also overhauling its energy policy following last year's nuclear crisis.

Recently, his government has also come under pressure from territorial disputes with China and South Korea.

On Monday, the LDP's chief, Sadakazu Tanigaki, said he would not run in his party's internal leadership election on Sept. 26. Several LDP veterans, including former Prime Minister Shinzo Abe and ex-Defense Minister Shigeru Ishiba, appear to be eyeing the party's leadership post.

Japan's political landscape is in flux amid the emergence of another political party led by Osaka Mayor Toru Hashimoto, a 43-year-old charismatic politician with populist and nationalist views.

His party, which is named the Japan Restoration Party, is centered in the Osaka region, but could draw defectors from the DPJ and LDP and be a key player in any upcoming elections, experts say.

Vying against Noda in the DPJ leadership race are two former farm ministers, Michihiko Kano and Hirotaka Akamatsu, and a former internal affairs minister, Kazuhiro Haraguchi.