Japan's Prime Minister Shigeru Ishiba is set to dissolve parliament on Wednesday ahead of October 27 snap elections, banking on his political honeymoon and a fragmented opposition to lead his scandal-tainted party to victory.

His Liberal Democratic Party (LDP) has governed Japan almost uninterrupted for decades and is almost certain to be re-elected.

But Ishiba, named prime minister just last week, wants to shore up his mandate to push through policies including beefing up spending on defence and on poorer regions hit hard by Japan's demographic crisis.

He replaced Fumio Kishida, whose government had suffered from record-low approval ratings due partly to slush fund scandals that stoked distrust in the LDP and voter discontent over rising prices.

Ishiba's backers hope that the self-confessed defence "geek" and an outspoken critic of the LDP establishment will boost the party's popularity.

By dissolving parliament now, the 67-year-old wants to put his party to the test before his "honeymoon" period ends, said Yu Uchiyama, a political science professor at the University of Tokyo.

"It makes sense that he wanted to call a snap election as soon as the 'face' of the party changed, while the momentum is still there", he told AFP.

Uchiyama added that Ishiba also wanted to catch opposition forces flat-footed as the LDP's foes remain undecided about how to coordinate with each other in the election.

But the prime minister's decision to call a snap election this early was also criticised for contradicting his previous vows to prioritise facing the opposition at parliament.

Some voters saw it as a disappointing sign that he had "yielded to the pressure within his party" to dissolve parliament for political gain, Uchiyama said.

Over the weekend, Ishiba announced that the LDP will not endorse some disgraced party members implicated in the political funding scandal in the upcoming election.

The announcement reflected his desire to demonstrate to the public that he can be "strict", and "likely regained public trust in him a bit", Uchiyama said.

Fresh promises

A former defence minister known for his in-depth security policy knowledge, Ishiba has backed the creation of a regional military alliance along the lines of NATO.

He said the security environment in Asia was "the most severe since the end of World War II", and had warned in his first policy speech Friday that "today's Ukraine could be tomorrow's East Asia".

Japan is also facing a looming demographic crisis as its population ages and the birth rate stays stubbornly low, a situation that Ishiba called a "quiet emergency" on Friday.

He said his government would promote measures to support families such as flexible working hours.

Ishiba has also pledged to "ensure Japan's economy emerges from deflation", and wants to boost incomes through a new stimulus package as well as support for local governments and low-income households.

It also pledges to allow married couples to maintain separate surnames.