About half of the money spent in competitive Senate races this cycle came from groups that do not have to disclose their funders, according to the new report.
Of the $190 million spent in 10 races this year, $90 million was spent by 501(c) nonprofits, which don't have to disclose where funding comes from, while the rest of the spending came from super PACs, according to the the report.
The study looked at contests in Arizona, Florida, Indiana, Missouri, Montana, Nevada, North Dakota, Ohio, Virginia and Wisconsin and casts a critical eye toward the post-Citizens United phenomenon of unlimited political spending by outside groups--"dark money," as the group calls it.
"The effect of Citizens United has been to deny the public the opportunity to learn the identity of sponsors behind a large percentage of political ads," said Public Citizen's Lisa Gilbert in a statement. "It is likely that the donors that have been kept secret are those that would be the most controversial, such as corporations seeking favors. Until Congress or agencies mandate disclosure, we will not know the truth. "