Bloomberg BNA:

Undisclosed donors provided nearly half of the more than $20 million in outside campaign spending in three closely watched, current congressional election races, a Bloomberg BNA review of Federal Election Commission reports found.

Bloomberg BNA looked at FEC independent expenditure reports filed in the special election campaigns for U.S. House seats for Georgia, Montana and South Carolina. These races, for seats vacated by Republican lawmakers who joined President Donald Trump’s administration, are being treated by both Republicans and Democrats as important bellwethers reflecting the mood of the electorate following Trump’s first months in office and have drawn record campaign spending.

Of a total $20.7 million, so far, in independent expenditures of $50,000 or more in the three House races, just under $4 million—nearly 20 percent—is from trade associations and nonprofit groups that disclose none of their donors.

This direct spending by non-disclosing nonprofits is only part of the story, however. Much of the rest of the outside campaign spending has come from super political action committees receiving large contributions from nonprofits or other entities that keep their original donors under wraps.