...is a House of Representatives election as soon as possible.
--because he thinks the Democratic Party of Japan can become the new majority party.
The personal relationships between Liberal Democratic Party (LDP) district representatives, around half of whom are 2nd, 3rd and even 4th generation representatives of their electoral districts, and their many, many clients are probably too high barrier for any challenger to leap over--at least until the next round of redistricting.
--because he wants to be prime minister.
Even if the political opposition prevents the LDP/New Komeitō coalition from winning a majority of seats, forcing the LDP/New Komeitō into negotiations establishing a grand coalition with the DPJ, opening the door for Ozawa to demand the premiership, it is doubtful that Ozawa has the physical stamina to be the nation's leader.
(All of which should not be interpreted to mean Ozawa does not desire either of the above electoral outcomes. Of course he believes that winning outright or keeping the ruling coalition from winning a majority would be just grrrrreat!)
Ozawa wants an immediate election in order for the DPJ to win enough seats to rob the current ruling coalition of its 2/3 majority in the House of Representatives. Winning 1/3 +1 seats, even if it requires continued collaboration with the Socialists and the Communists, would be a worthwhile achievement--because it would terminate the current coalition's ability to override the actions or inactions of House of Councillors. Denying the ruling a coalition of override power would force the ruling coalition to good faith (i.e., DPJ-dominated) negotiations about all legislation and appointments--something the ruling coalition has failed to do so far and--as the nomination of former Finance Ministry Vice Minister Watanabe Hiroshi to be Vice-Governor of the Bank of Japan demonstrates--the ruling coalition continues to fail to do.
A modest and proximate goal, 100% guaranteed to happen. The DPJ would need to take only around 20 seats from the ruling coalition--in an election that would be testing the abnormal Koizumi Jun'ichiro-engineered LDP House of Representatives majority of 2005.
Ocha-no-ko saisai (piece of cake).
So why do so many pundits keep floating a "the DPJ should be careful, not push too hard on getting its own way, because the party could come out looking obstructionist, costing it its chance at victory in the next election" nostrum?
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