I just noticed that the December 2009 issue of the FCCJ's Number 1 Shimbun had a report entitled "Jake Adelstein and Tokyo Vice" by Gavin Blair. The report mentions an Adelstein's claim that the Japan's largest yakuza group, Yamaguchi-gumi, is supporting the DPJ.
Something that definitely falls into the controversial category is Adelstein’s claim that the Yamaguchi-gumi – Japan’s largest criminal organization – switching its traditional support for the LDP over to the DPJ about a year before the Democrats’ election victory. While somehow reminiscent of the Murdoch press in the U.K. shifting allegiance from the Conservatives to Labour (and recently back again) when either side emerged a likely winner, there may have been more to it than simply backing the winning horse, according to Adelstein. He quoted two Japanese weeklies – Asahi Geino and Shukan Jitsuwa – as reporting that the switch to the DPJ had been decided at top-level Yamaguchi-gumi meetings.According to this passage, Adelstein seems to have quoted only vulgar weekly journals such as Asahi Geino and Shukan Jitsuwa, whose credibility is questionable. Also, Gavin Blair, the writer of the article, reported that the alleged Yamaguchi-gumi's support was definitely controversial. However, as long as I know, it is not controversial. Japanese news media already reported it in 2007. I remember this, because it was a big surprise for me that the whole Yamaguchi-gumi decided to support a single political party. There had been a few times that Japanese media reported that individual politicians had connections with some members of yakuza organizations. When such connections were revealed and reported by the media, they were always big scandals for the politicians that may lead to political death of them. Therefore, it was a big surprise that it was revealed that a whole yakuza organization officially decided to support a single political party.
Since Japanese media do not keep their articles on the web for a long period, the news articles have already been removed from the web. However, I could find a cache of an article. According to the article, a member of Yamaguchi-gumi was arrested for buying votes for a DPJ candidate in the election of the House of Councillors in 2007, and it was revealed that the headquarter of Yamaguchi-gumi had issued an official notice that ordered member organisations to support the DPJ.