The crowd of 800 people were younger than those who typically attend political rallies in Japan. But they had gathered in the shadow of a smoking volcano to hear a populist upstart in Sunday’s parliamentary elections whose heated campaign speech would sound familiar to voters in the United States or Europe.

They burst into cheers when Sohei Kamiya climbed to the top of a campaign truck decorated in the orange colors of his fledgling political party, Sanseito. Grabbing a microphone, he told them that Japan faced threats from shadowy globalists, lawbreaking foreigners and a corrupt domestic political establishment that was stifling the younger generation with taxes. His solution: a nationalist agenda that he calls “Japanese First.”

“Japan must be a society that serves the interests of the Japanese people,” Mr. Kamiya told his applauding audience.

Mr. Kamiya founded the party and is one of its two sitting members in the Upper House. Elected to a six-year term in 2022, he is not on the ballot himself this year. But he has crossed Japan to campaign on behalf of Sanseito’s 54 candidates, a large number that reflects the new party’s big ambitions.