The last House of Councilors election in 2007 heralded a change of regime, with the then ruling Liberal Democratic Party losing control of the chamber. That loss was followed by the swift resignations of two LDP prime ministers, before a crushing defeat in the House of Representatives poll last August saw the long dominant party ejected from power.

Fast-forward to the House of Councilors election set for July 11, and observers agree this year's vote — the first to be held since the Democratic Party of Japan took power last summer — could again drastically shape politics.

"The focus is on whether the DPJ-led coalition can control a majority in the Upper House," said Rei Shiratori, president of the Institute for Political Studies in Japan.