TOKYO, Sept 7, 2012 (AFP) - Japan's prime minister was on Friday set to declare his intention to run for re-election as party leader, with the way to victory apparently clear after his main potential rival stepped aside.

Yoshihiko Noda looked set to be the sole candidate for the job of president of the ruling Democratic Party of Japan (DPJ), a post that currently comes with the prime ministerial portfolio by dint of its being the largest party.

His telegenic environment minister, Goshi Hosono, had been mulling a crack at the job, which would have made him Japan's youngest ever prime minister -- and the seventh man in the role in six years -- but ruled himself out.

"I just met the prime minister and told him I myself will not run in the leadership election," 41-year-old Hosono told reporters.

Hosono had increasingly appeared to be a good choice for the flailing DPJ, who look set to take a beating in the general election, expected in the autumn.

Noda's copybook has been blotted by his pushing through a politically toxic, but economically necessary, doubling of the sales tax, and many in the party fear an election under the current leader is unwinnable.

But Hosono, whose star has risen with what is seen as his capable handling of his brief, which includes dealing with the aftermath of the Fukushima disaster, said he wants to focus on his current job.

"I cannot cast aside this task as the person responsible on the government's side," he said. "I just couldn't help thinking about Fukushima."

Nominations for the leadership close on Monday. At least one party member is reportedly still considering a bid, but Noda is widely expected to be unopposed, leaving him in the driving seat for the general election.

The main opposition Liberal Democratic Party (LDP) is currently just ahead of the DPJ in opinion polls -- although neither party enjoys overwhelming voter confidence -- and is also in the throes of a leadership campaign.

Several candidates, including conservative former prime minister Shinzo Abe, are vying for the post of party president, which is now held by Sadakazu Tanigaki.

Observers say Noda's factionally-divided DPJ is likely to suffer at the hands of voters disappointed by their lacklustre three years in office.

The DPJ came to power in 2009 after five decades of almost unbroken rule by the LDP. But its once-radical agenda was largely jettisoned.