So-called “dark money” groups—nonprofit organizations that spend on campaign advertising but are not required to reveal their donors—spent $300-million on the presidential and congressional elections, 80 percent of it backing Republicans, CBS News reports, citing a study by watchdog group the Sunlight Foundation.

The activity by 501(c)(4) social-welfare organizations amounted to nearly a quarter of the money spent by independent political entities on this year’s campaign, according to the foundation, which reviewed Federal Election Commission spending disclosures. Another watchdog organization, Public Citizen, found that 86 percent of campaign spending by outside groups paid for negative attack ads.