"While dark money has mostly been on the side of Republicans in recent election cycles, a relatively new liberal outside group has dominated dark money spending during the 2018 midterms, effectively injecting millions into crucial U.S. Senate races," the group said on Wednesday.
Overall, the nonprofit group contributed more than $40 million to help Democratic candidates, which marks the most of any dark money group this election cycle, the report said.
In July, the Democratic delegation in the US Senate sent a letter to US Treasury Secretary Steven Mnuchin and Internal Revenue Service (IRS) Commissioner David Kautter urging them to reinstate a requirement for organizations to identify the source of their contributions to prevent illegal donations in the US elections.
READ MORE: ‘Dark Money’ Victory: US Lobby Groups No Longer Have to Disclose Donors
On July 16, the US Department of the Treasury and IRS announced that the IRS will no longer require certain tax-exempt organizations to file personally identifiable information about their donors as part of their annual return.
Critics have said, in turn, that this rule change will make it easier for "dark money" to influence US elections.