Yesterday’s New York Times featured Fine Print is Given Full Voice in Campaign Ads, which considers the “stand by your ad provision of BCRA.” Though Congress intended the provision to curb negative ads, the provision does not provide a means for distinguishing between negative and other ads.
The article quotes reformer Fred Wertheimer as follows:

“Who, pray tell, is going to decide which ads are positive and which ads are negative?” said Fred Wertheimer, president of Democracy 21, a campaign reform advocacy group.

In response, Joe Birkenstock here on the election law listserv notes what he sees as an inconsistency in Wertheimer’s position: Wertheimer says it is impossible to tell a positive ad from a negative ad, but he supports the definition of “federal election activity” in BCRA that requires identifying ads that “promote, support, attack, or oppose” a candidate. See also Brian Svoboda’s thoughts.