"Off The Record"
But why, inquiring minds want to know, do campaigns release opposition research “not for attribution?” Ana Marie Cox has some discussion but I think it can actually be boiled down pretty simply. Roughly speaking, “Hillary Clinton Sucks” is a good story for Barack Obama whereas “Obama: Clinton Sucks” is a good story for John Edwards.
Negative campaigning is difficult in the context of a primary campaign, since your only rivals dangerous enough to be worth attacking are people who are generally well-liked by the target audience of the electorate. You want negative stories about your rivals to get written, but you don’t want stories to be written about your negative attacks on your rivals.