WASHINGTON – Just 15 groups account for three-quarters of the anonymous cash flowing into federal elections since the Supreme Court paved the way for corporate and union money in candidate races eight years ago, according to a new report provided first to USA TODAY.

The analysis by Issue One, a group that supports greater campaign-finance regulation, finds many of these groups remain big players in the 2018 midterm elections, although the sources of their money remain largely hidden from public view.

“If you have the guts to play in politics or attack a candidate running for office, you should have the courage to put your name on it,” Issue One CEO Nick Penniman said in a statement about the report, “Dark Money Illuminated.”

Outside money in elections has exploded since the high court’s blockbuster Citizens United ruling in 2010, allowing corporations and unions to spend in candidate elections. Groups that don’t disclose their donors spent more than $800 million between Jan. 1, 2010 and December 2016, with $600 million coming from 15 organizations, Issue One’s analysis found.

Those groups range from the U.S. Chamber of Commerce and Planned Parenthood Action Fund to the National Rifle Association and Americans for Prosperity, the grassroots arm of the influential political network associated with billionaire industrialist Charles Koch.

The report chronicles how hard it is for average voters to determine who’s behind the ads that appear on their television screens, land in their mailboxes or pop up on their mobile devices during the height of political campaigns.

The group’s researchers spent a year combing through available records – Federal Election Commission filings, tax returns, corporate filings and the like – in an attempt to piece together donors’ identities. They found 402 donors to these groups but could only pin down information on $1 of every $9 raised by these organizations.

That’s because much of the money flowing into politically active nonprofits comes from other nonprofits that don’t disclose their donors, either.

The researchers collaborated with the nonprofit news organization ProPublica on a publicly available database of the information they found.

Intense battles have been waged over disclosure of non-profit donors.

On Tuesday, a three-judge federal appeals court ruled that a charitable arm of Koch's network, the Americans for Prosperity Foundation, must disclose its contributors to California's attorney general. The legal fight could end up before the Supreme Court.

Issue One also is using its report to call for Congress to pass greater transparency measures, such as a proposal to identify the groups’ top donors in the political ads they run on TV and social media.

It faces stiff headwinds in that quest.

The Honest Ads Act, one of the measures pushed by campaign-finance watchdogs, is stalled in congressional committees. And this summer, the Trump administration announced it would no longer require tax-exempt groups such as the NRA to identify their donors to IRS officials.

The names of these donors have not been publicly available, but until the administration’s action, groups were required to disclose the contributor information to the IRS on their annual tax returns. The agency has mistakenly released tax returns with donor information in the past.

Treasury Secretary Steven Mnuchin has said the IRS doesn’t need the donor information to enforce tax laws.

In a sign of the mood in the Republican-controlled Congress, Senate Majority Leader Mitch McConnell, R-Ky., hailed the administration’s action as protecting donors from harassment by "angry, left-wing activists” should their identities become public.