The allegations of cronyism have also sparked calls for Aso to resign, and are hampering Abe's bid to achieve his long-held ambition of changing the pacifist constitution.
The demonstration comes amid claims that Abe was involved in the sale of state-owned land at a huge discount to a nationalist school operator which has ties to his wife.
Former National Tax Agency chief Nobuhisa Sagawa raises his hand to testify under oath during a House of Representatives Budget Committee meeting, on March 27, 2018.
Obama and Japanese Prime Minister Shinzo Abe also visited Hawaii's Pearl Harbor in a show of reconciliation between the two former wartime foes.
However, the official declined to answer detailed questioning about how and when documents were altered, saying he was under criminal investigation.
The Finance Ministry has conceded that 14 documents related to the cut-price land transaction were altered between late February and April previous year after officials at the Financial Bureau ordered its regional bureau in Osaka to do so and insisted Sagawa having played a key role.
Sagawa was head of the finance ministry department that oversaw the sale in 2016, and was promoted to head Japan's tax agency the following year.
Several times previous year, Sagawa told MPs the price of the land "was properly calculated" and stressed there was no evidence of political pressure in the sale.
Abe has apologized and promised an investigation into the sale, but has denied any personal wrongdoing.
Abe said Sagawa's words reinforced his repeated denials of giving instructions to falsify documents related to a dubious land sale.
The premier has been on the ropes in recent weeks amid a scandal over the cut-price sale of government land to an Abe supporter.
A key figure in a document-tampering scandal involving Japan's Finance Ministry on Tuesday refused to testify on exactly how the ministry's Financial Bureau had deliberately altered the documents.
The suspected scandal and cover-up have slashed Abe's support rates and clouded his chances of a third term as ruling Liberal Democratic Party (LDP) leader in a September party vote.
Those who said they supported Abe slid to 32.6 percent, down 11.7 percentage points from a month ago, while those who said they did not rose 13.2 percentage points to 54.9 percent.