Last week, North Carolina mega-donor Art Pope announced his endorsement of Sen. Marco Rubio’s (R-FL) presidential campaign. Pope, who said he personally met with nearly every GOP primary contender before making his choice, is a significant get for Sen. Rubio. A member of the Koch brothers’ political network, Pope has deep pockets and is credited with funding the effort that led to a conservative takeover of the Tar Heel State.

Pope said he intends to contribute to Sen. Rubio’s presidential campaign coffers, but refused to elaborate any further on his plans, particularly whether he would use his vast resources to support outside groups aiding Sen. Rubio’s campaign. Like most of the candidates in 2016, Sen. Rubio has a super PAC—which can accept unlimited donations—supporting his campaign. Unlike the other candidates, most of Sen. Rubio’s outside support so far has come from a 501(c)(4) nonprofit social welfare organization known as the Conservative Solutions Project, Inc. (CSP) that doesn’t need to reveal its donors. Pope is rumored to have invested in a single-candidate nonprofit before, leading to questions whether he’ll donate to the Rubio-supporting nonprofit.

In 2014, the Washington Post reported that Pope was said to be a donor to Carolina Rising, a nonprofit that spent millions to help get Sen. Thom Tillis (R-NC) elected. Tax documents show that Carolina Rising had only two donors that year, including one who provided 98.7% of its $4.88 million in revenue. After Citizens for Responsibility and Ethics in Washington (CREW) filed an IRS complaint against Carolina Rising in October 2015 for illegally operating for Sen. Tillis’ private benefit, Pope denied that he or his company gave the big $4.82 million donation.

Notably, Pope did not deny contributing any money—he could still be the other donor. It was later revealed that another dark money nonprofit, Crossroads GPS, was Carolina Rising’s primary funder. It is also possible he contributed to Crossroads GPS, which in turn donated to Carolina Rising, but that can’t be known because Crossroads GPS isn’t required to reveal its donors either.

If Pope does decide to contribute to CSP, he would be supporting a group that exists primarily to benefit Sen. Rubio’s presidential ambitions, despite its claim to be a “social welfare” organization. As with Carolina Rising, CREW filed an IRS complaint against CSP in October 2015, alleging that the organization has violated its tax-exempt status by spending millions on advertisements to support Sen. Rubio’s presidential campaign.

Unfortunately, the public may never learn if Sen. Rubio’s latest millionaire backer is backing his single-candidate 501(c)(4), since the organization can keep its donors secret. Of course, that’s largely the point of using a nonprofit to boost a candidate: anonymity for donors.

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