An upstart party is gaining support ahead of elections in Japan by railing against a 'silent invasion' of immigrants, pushing the government to tackle fears about foreigners, a topic once confined to the political fringe.

Birthed during the COVID-19 pandemic, the party, Sanseito, is widening its appeal with a 'Japanese First' campaign ahead of this weekend's upper house vote.

And while polls show it may only secure 10 to 15 of the 125 seats up for grabs, it is further eroding the support of Prime Minister Shigeru Ishiba's shaky minority government increasingly beholden to opposition parties as it clings to power.

"In the past, anyone who brought up immigration would be attacked by the left. We are getting bashed too, but are also gaining support," Sohei Kamiya, the party's 47-year-old leader, said in an interview.

"The LDP and Komeito can't stay silent if they want to keep their support," Kamiya added, referring to Ishiba's Liberal Democratic Party, which has ruled Japan for most of the past seven decades, and its junior coalition partner.

Kamiya's message has grabbed voters frustrated with a weak economy and currency that has lured tourists in record numbers in recent years, further driving up prices that Japanese citizens can ill-afford, political analysts say.

The fast-ageing society has also seen foreign-born residents hit a record of about 3.8 million last year, although that is still just 3 per cent of the total population, a fraction compared to numbers in the United States and Europe.

Kamiya, a former supermarket manager and English teacher, says he has drawn inspiration from US President Donald Trump's "bold political style".

It remains to be seen whether he can follow the path of other far-right parties with which he has drawn comparisons, such as Germany's AFD and Reform UK.

Yet the ingredients are there, said Jeffrey Hall, a lecturer at Tokyo's Kanda university who has studied Japan's right-wing politics, pointing to their online following, appeal among young men and warnings about immigration eroding indigenous cultures.

"Anti-foreign sentiment that was considered maybe taboo to talk about so openly is now out of the box," Hall said.

With immigration emerging as a top election issue, Ishiba this week unveiled a new government taskforce to fight "crimes and disorderly conduct" by foreign nationals and his party has promised to pursue "zero illegal foreigners".

Polls show Ishiba's ruling coalition is likely to lose its majority in the upper house vote, in a repeat of elections last year in the more powerful lower house.

While he is expected to limp on, his government may have to broaden its coalition or strike deals with other parties on policy matters, analysts say.