Opposition parties are poised to step up their offensive against Prime Minister Shigeru Ishiba, who some say has dug his own grave by giving gift certificates to new lawmakers of his Liberal Democratic Party.

Ishiba, already hurt by low public support, has found himself in a bind ahead of this summer's Upper House election as the scandal shakes his image as a "clean politician," one of his key selling points.

With Ishiba rapidly losing support within the ruling camp, the political situation has become volatile, and the opposition is examining when best to demand his resignation.