No presidential candidate has relied on “dark money” as much as Marco Rubio has this year.  

The Conservative Solutions Project has spent nearly $8.5 million on TV ads, making it the second biggest advertiser so far in the entire presidential race according to Kantar Media data. The group is paying for ads in early-voting states that feature Rubio, though they usually avoid openly calling for his election. As a 501(c)(4) organization, the Conservative Solutions Project does not have to disclose its donors. But it is also expected to further the “social welfare,” not simply promote a single candidate.  

Other candidates are benefiting from 501(c)(4) organizations — colloquially referred to as dark money groups — but, as Andrew Prokop of Vox notes, only Rubio has one that is airing TV ads. By contrast, Right to Rise Policy Solutions, a 501(c)(4) with ties to Jeb Bush, has focused on issuing policy papers. Rubio has only just started running his own campaign ads, but Conservative Solutions Project has been airing spots for weeks that feature footage of the Florida senator. Here are two of the CSP’s advertisements, both built around Rubio speeches, one on domestic policy, the other on foreign affairs.

Citizens for Responsibility and Ethics in Washington (CREW) first spotted the fact that CSP’s advertising forms refer specifically to Rubio; CREW subsequently filed a complaint to the IRS, claiming that this political activity by the Conservative Solutions Project violated 501(c)(4) tax law. Fellow watchdog organizations Campaign Legal Center and Democracy 21 have followed suit, filing a separate complaint and urging the Department of Justice to investigate the Conservative Solutions Project, arguing that is simply set up to promote a presidential candidate — which would indeed be a violation of IRS rules.

This suggests that the Conservative Solutions Project may be primarily aimed at promoting Rubio as a presidential candidate. A New York Times editorial called the tactics “one of the most brazen” abuses of the campaign finance system so far in the presidential race.  

The Conservative Solutions Project is not alone. American Encore, another nonprofit controlled by a political operative who is backing Rubio, is now running TV ads in Iowa attacking Ted Cruz for his record on electronic surveillance issues. The commercial, which opens with footage of the Paris terror attacks, compares Cruz to Barack Obama and warns that he voted to weaken “our ability to fight terrorists.” American Encore, which ran also ads in the 2014 Arizona gubernatorial elections, does not disclose its donors.  

As Prokop notes, Rubio has a history of being close to his top donors, such as Florida businessmen Norman Braman and Max Alvarez. But because of this group’s exploitation of its IRS status, we cannot know who might be funding the main source of pro-Rubio ads so far. Rubio has said that he wants “sunlight” that illuminates the identity of donors and spending. But instead, he is benefiting from dark shadows that keep voters from seeing who is helping him on his quest to win the White House.