Open Secrets on how one non-disclosing group funds another:

One of the largest and most prolific politically active nonprofits in the country was virtually the sole backer of Carolina Rising, the group that spent $4.7 million on ads supporting now-Sen. Thom Tillis’ successful run to unseat incumbent Democrat Kay Hagan in 2014. That was nearly 100 percent of Carolina Rising’s spending, as OpenSecrets Blog wrote last month.
Crossroads GPS not only provided 98.7 percent of Carolina Rising’s funds, but it also spent tens of millions of its own money in North Carolina and elsewhere in the midterms, according to tax documents obtained by the Center for Responsive Politics.