Just a few weeks out from the November midterm elections, a new report revealed on Tuesday that over a quarter of the nearly $1 billion spent so far has been paid for by outside groups, the majority of which do not disclose their donors.

According to the Wesleyan Media Project analysis (pdf) of Kantar Media/CMAG data, the top three outside spenders—The Republican Governor’s Association, Crossroads GPS, and the NRA Political Victory Fund—all largely back Republican candidates in upcoming congressional and gubernatorial races.

Crossroads GPS, a 501c4 started by conservative strategist Karl Rove, outspent all others, pouring $3.8 million into advertising for GOP candidates in key Senate races in Arkansas, Arizona, Colorado, Iowa, Kentucky, Louisiana and North Carolina.

The volume of advertisements favoring Democratic candidates slightly outnumbers those favoring Republicans. However, researchers note that because Republicans are “more reliant on outside groups,” they must pay more for advertisements.

“Because Republicans are relying on outside groups to pay for their ads, they are not getting as much bang for the buck as Democrats,” said Michael Franz, co-director of the Wesleyan Media Project. As Franz explains, “Sixty days before an election, candidates are entitled to the lowest unit rate from television stations,” however, that does not apply to Super PACs (Political Action Committee) and other outside funding groups.

“Still,” Franz reasons, “because political science research suggests outside spending can be more persuasive than ads from candidates, this strategy may pay off for Republicans.”