Opposition party Nippon Ishin no Kai on Tuesday decided to vote against a bill to revise the political funds control law in the House of Councilors.
Nippon Ishin, which had voted for the bill in the House of Representatives, is also considering submitting a censure motion against Prime Minister Fumio Kishida, president of the Liberal Democratic Party, to the Upper House.
The LDP aims to enact the bill at an Upper House plenary meeting Wednesday before the current parliamentary session ends on Sunday.
"We'll have to reject (the bill) if things remain as they are," Fumitake Fujita, secretary-general of Nippon Ishin, told reporters. He criticized Kishida for what he called a lack of governance.
In the Lower House, Nippon Ishin voted for the bill, drawn up in response to the LDP's slush fund scandal, following an agreement between Nippon Ishin leader Nobuyuki Baba and Kishida.
However, Nippon Ishin changed its mind after the LDP decided that during the current parliament session it would not revamp what are known as the research, public relations and accommodation allowances provided to lawmakers, even though reform was included in the agreement.
The bill, which went through amendments, calls for lowering the threshold for the disclosure of information on those who bought political fundraising party tickets from the current ¥200,000 ($1,265) to ¥50,000.
Under the bill, receipts for spending from what are known as policy activity funds, which political parties provide to member lawmakers, will be disclosed after 10 years, as sought by Nippon Ishin.