Official campaigning kicked off in Japan on Tuesday ahead of a December 14 election that Prime Minister Shinzo Abe has described as a referendum on his faltering "Abenomics" growth blitz.

"We are determined to win," Abe told hundreds of voters at his initial stump speech in the northern port of Soma. "I promise to make Japan a country that can shine again at the centre of the world."

The town lies around 40 kilometres (25 miles) from Fukushima Daiichi nuclear plant, which was sent into meltdown by the 2011 tsunami. Its fragile economy and ageing populace are a stark reminder of the challenges Abe's government faces as it struggles to kick-start Japan's lethargic economy.

More than 1,180 candidates nationwide were vying for 475 legislative seats in the powerful lower house of parliament, with Abe's Liberal Democratic Party seen as likely to cruise to a comfortable majority.

The 60-year-old premier still has two years left in his mandate, but he called for the vote in the wake of his decision last month to delay a planned sales tax hike to 10 percent next year.

A levy rise in April -- from 5 percent to 8 percent -- slammed the brakes on growth and pushed the country into recession during the July-September quarter.

The tax hike, aimed at tackling Japan's eye-watering national debt and huge public welfare costs, dealt a serious blow to Abe's plan to conquer years of growth-sapping deflation.

Ratings agency Moody's on Monday downgraded Japan's credit rating by one notch to A1, citing "rising uncertainty" over the country's debt situation and Abe's faltering efforts to kickstart growth.

But Deputy Chief Cabinet Secretary Hiroshige Seko brushed aside the downgrade, saying: "Our prime minister has clearly said he will firmly maintain the fiscal soundness target and map out concrete plans by summer next year to achieve the goal."

- 'Election without cause' -

While Abe's once soaring popularity has taken a hit, there is little in the way of credible opposition and observers said the election was likely aimed at fending off internal party rivals ahead of a leadership vote next year.

However, in the wake of several cabinet minister spending scandals, and with a sizable chunk of the public opposed to his plan to restart nuclear power, the ruling party may lose some of the 295 seats it held before parliament was dissolved in November 21.

Critics have derided the vote -- which will cost taxpayers about $500 million -- as "an election without a cause," but Abe has insisted the poll is necessary as a referendum on his controversial big-spending, easy-money policies.

"This is an election to judge the economic policies called Abenomics that we have been pushing," Abe said.

The prime minister's pro-business policy blitz, launched in late 2012, has boosted the Japanese stock market and driven down the yen, cheering investors and exporters.

But critics say the programme has inflated inequality, creating low-wage jobs and limited benefits, while adding to Japan's debt -- already one of the heaviest burdens among wealthy nations at more than twice the size of the economy.

For voters, the state of the economy, streamlining the size of Japan's parliament, and better child care programmes to lure more women into the workforce were the top three issues, according to a survey published by the Asahi newspaper on Monday.

Some 34 percent of surveyed voters said they would vote for the LDP, followed by 13 percent for the main opposition Democratic Party of Japan.

The survey showed that Abe's cabinet maintained an approval rating of 40 percent, nearly flat from 39 percent seen in the same survey taken a week ago.

The opposition has criticised the conservative leader for his decision to expand the role of the Japanese military and his visit to the controversial Yasukuni war shrine late last year, as well as efforts to restart nuclear power after the Fukushima crisis.