TOKYO -- Japanese Prime Minister Shinzo Abe's government struggled Tuesday to contain the fallout from document doctoring linked to a controversial land deal, trying to spare a beleaguered finance minister from resignation while shifting blame to career bureaucrats.

Opposition lawmakers stepped up their attacks, demanding that Nobuhisa Sagawa, a former tax chief who quit over the alterations, submit to parliamentary questioning. While even some ruling party members increasingly saw Sagawa's Diet appearance as inevitable, the standoff is expected to intensify over the fate of Finance Minister Taro Aso.

The government's stance is that the rewriting was done internally by Finance Ministry bureaucrats and that top government officials like Abe and Aso thus should not be held accountable. Abe and his deputies hope this narrative will help bring the controversy to a quick close.

"It is my job as minister to get to the truth of the matter and prevent this from happening again," Aso told a news conference Tuesday, again dismissing speculation that he will step down.

The ministry has admitted changing 14 documents relating to the sale of public land to Moritomo Gakuen, a nationalist private-school operator with ties to Abe and his wife Akie for far below market value. Discrepancies between the original versions and those released to lawmakers last year came to light in a news report earlier this month.

Aso has explained that the documents were altered to bring them into line with Sagawa's testimony to lawmakers on the land sale to Moritomo Gakuen. Sagawa headed the Finance Ministry's Financial Bureau when the changes were made.

Chief Cabinet Secretary Yoshihide Suga also said Aso need not step down. The government will carry out a "thorough investigation" into "why this happened and how we need to improve," said Suga, who deemed answering these questions "extremely important."

If calls for Aso's exit gain momentum, "there will be nothing to stop growing calls for political responsibility," an official in Abe's Liberal Democratic Party said, implying that Abe, too, could come under pressure. The government is intent on using the LDP's numerical advantage in the Diet to bury the scandal amid debate on the budget. No major national election is slated for 2018, meaning that public opinion will have plenty of time to recover.

Nobuhisa Sagawa, then head of the National Tax Agency, flanked by Prime Minister Shinzo Abe, left, and Finance Minister Taro Aso.   © Kyodo

But if the public views Sagawa's resignation as a mere ploy to bring the scandal to a swift end and the cabinet's approval rating takes a dive, Aso will have little choice but to leave.

Some in the LDP have floated the possibility of the opposition promising to help pass key legislation related to the fiscal 2018 budget in exchange for Aso's resignation. While the budget itself will be automatically approved 30 days after being sent to the upper house for debate, and thus stands to pass before the fiscal year is out, related legislation has no such safeguard.

The opposition has made "legislative debate extremely difficult," a senior LDP official said. And while the government has already taken into account the Moritomo Gakuen scandal will likely bring down approval ratings, it would still like to dispel the cloud hanging over its legislative agenda. A Diet deadlock could affect not only budgetary matters, but also actual governing. Two deputy governors' seats at the Bank of Japan could sit empty after their current occupants depart next Monday, and whether Aso will make it to a meeting of Group of 20 finance ministers and central bankers in Argentina that day is unclear.

Yet Aso's departure would deal a major blow to the Abe government. The finance minister has also served as deputy prime minister since Abe returned to power in 2012 and heads ongoing economic dialogue with the U.S. The government has maintained that he would be a tough act to follow, given that he is a valuable partner to the prime minister in both internal and foreign affairs.

For now, the most pressing question is whether key figures in the document scandal will be compelled to testify before lawmakers. The ruling party has so far ignored opposition demands for calling in Sagawa and Akie Abe. But Wataru Takeshita, chairperson of the LDP's General Council, told a news conference Tuesday that Sagawa should be called "if it becomes clear he was ultimately responsible" for the alterations. Natsuo Yamaguchi, head of the LDP's junior coalition partner, Komeito, said it is "conceivable that we could call in [Sagawa] if necessary."

Some in the ruling party have gone even farther. "Do we want politicians who simply force responsibility onto mere bureaucrats?" asked Seiichiro Murakami, a former state minister for regulatory reform, on Tuesday. "The prime minister needs to do some serious soul-searching."

"It will soon be time for Prime Minister Abe to make a reasoned decision based on a broad view" of the circumstances surrounding him, Murakami said. He referenced former Prime Minister Noboru Takeshita, who resigned over a Recruit bribery scandal in the 1980s.