The U.S. Treasury will no longer require certain tax-exempt groups to list their financial donors to the Internal Revenue Service, a move praised by conservatives and blasted by those fearful of dark money's sway over American politics.

Sen. Jon Tester criticized Monday's late announcement by Treasury Secretary Steven Mnuchin.

“This is the swampiest, darkest, dirtiest decision,” Tester said. “We need more transparency in our campaigns, not less.”

The change protects the privacy of donors who make dark money contributions of more than $5,000 to politically active, tax-exempt organizations, including Planned Parenthood, the National Rifle Association and the Democratic Socialists of America.

Nonprofit groups that receive tax-deductible donations, such as charities, will still have to report the names of their larger donors, according to CNN.

While supporters see Mnuchin's announcement as a win for free speech, critics say the rule will make it easier for politically active lobbies to conceal large contributions, including those from foreign countries.

Tester, a longtime opponent of dark money and anonymous donations, blasted the rule change and vowed to sponsor legislation requiring the disclosure of campaign contributions.

“When special interests try to hide in the shadows, I will drag them into the light,” Tester said.

While the tax returns of nonprofit groups are considered public record, information on donors' names was never subject to public disclosure, according to Mnuchin.

“Americans shouldn’t be required to send the IRS information that it doesn’t need to effectively enforce our tax laws, and the IRS simply does not need tax returns with donor names and addresses to do its job in this area,” he said in a statement.

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