For a candidate who is double-listed as both a district candidate and a proportional bloc seat candidate, he or she can win a proportional bloc seat if and only if he or she garners at least 10% of the valid votes (spoiled ballots do not count) in the district contest.Could there possibly be a more transparent "I hate Socialists and Communists so let us really mess them up by making it too risky for their most promising candidates to run for district seats because to do so would risk their eligibility in the proportional election" rule? Not that the Democratic Party of Japan would complain, of course. Such a rule would be a goad to the Communists in particular to stop running candidates in the districts, splitting the progressive vote and by so doing being the facilitators of continued Liberal Democratic Party dominance.
Verily, I hope the LDP has been inviting the Communists to their end-of-the-year bashes these past few years out of gratitude for the part the Communists have played in keeping Japan a one party state - even though it was not the one party the membership of the JCP intended. Maybe after both the LDP and the Communists get thrashed on August 30, they will meet up at some hotel to commiserate and reminisce how each helped perpetuate the other in "the good old days."