Already in this election cycle, nonprofit groups using the cover of "issue advertisements"—thinly disguised attacks or promotions of a particular candidate—have spent about $30 million on general election ads, according to a new study. But only about $4.4 million of buys have been disclosed to the Federal Election Commission.

That’s because the issue ads, also known as "electioneering communications," must be disclosed to the FEC only when they air on TV or radio in the weeks immediately preceding an election. It's a quirk in the election law that offers another way for deep-pocketed donors to ...