The news is full of plots and counterplots, of Democratic Party of Japan Secretary General Ozawa Ichiro trying to cling to power, of Prime Minister Hatoyama Yukio having double-crossed Ozawa last week on Futenma.
There is likely some truth in all of these assertions and many more. The press has a story to tell -- "the Democratic Party is riven with factional infighting, with pro-Ozawa and anti-Ozawa forces at each others' throats" -- so members of the DPJ leadership have probably shrugged their shoulders and decided to go along it.
Nevertheless, the simplest analysis is probably the most likely to be true.
1) Everyone is agreement that Kan Naoto was, is and will be the relief pitcher for Hatoyama.
2) For the party to have a real leadership fight just before the election would be deeply damaging to the party's chances of capitalizing on the departures of Hatoyama and Ozawa. So the party needs to put on the semblance of a leadership fight, with only the vaguest outlines of a contest going on (a campaign period of just one day; no participation by the prefectural party chapters...).
3) No one wants to upset the delicate balance of forces within the party. However, for appearance's sake, somebody has to run against Kan. Tarutoko Shinji, a politician no one had ever heard of, has been drafted to provide the alternative. After Kan wins, the party has a new leader and Tarutoko can add "finished in second place in the voting for party leader on June 4, 2010" to his resume.
As a side note, the final tallies of votes for Kan and Tarutoko are likely not make any sense in terms of group voting. Neither Ozawa nor the party has an interest in giving a clear sense of how many legislators take their orders from him. Ozawa may indeed ask half of the 150 members of his group to vote for Kan and half to vote for Tarutoko, just to start generating a "Ozawa has lost his grip" story line in press reports.