Prime Minister Taro Aso, whose approval rating has slumped, may still have the support of bondholders due to perceptions that the current administration is more reluctant to sell debt than a new government formed by the opposition might be.

"As a bond investor, I want the ruling party to continue," said Yuuki Sakurai, general manager of financial and investment planning at Fukoku Mutual Life Insurance Co., which manages the equivalent of $54 billion in assets. "The current minister of finance is very keen on protecting the balance sheet of Japan. If the opposition wins, they will try to put more cash into the economy, putting the fear back into the bond market about issuance."

Bond investors are concerned that the Democratic Party of Japan, the largest opposition force, would boost spending, adding to the ruling Liberal Democratic Party's ¥10 trillion in fiscal measures announced in December and additional stimulus plans currently in discussion. The daily Mainichi Shimbun reported Friday the package may total ¥20 trillion.