Mike Wittenwyler, an attorney who represents the Club for Growth and other political groups, says such a law would be unconstitutional. Specifically, it would run counter to a landmark 1976 Supreme Court ruling that recognized a fundamental difference between ads that expressly advocate for candidates by using specific words, such as "vote for" or "vote against," and ads that avoid such terms.

That does not mean, he says, that the state can't regulate issue ads. What it could do, he suggests, is set up a framework that mimics existing regulations for federal elections. Currently, the federal government requires that all ads that mention a candidate within 60 days before an election disclose the spending.

"If the state adopted a statute like that, it depends on what it looks like, but (it might pass constitutional muster)," he says.

While such a system would force groups like WMC and the Club for Growth to immediately disclose how much they have spent on TV ads and other communication, it might not force them to disclose their donors. As is the case in federal elections, many big groups report that none of the money they received was intended to specifically fund ads, and therefore does not have to be disclosed.