they don't give a link to the poll in this caucus post, so i don't know how the question was worded. but i expect that part of the reason that "attacking their opponents" did so badly is because the poll probably calls it "attacking their opponent."
the powers that be make this distinction between "positive" and "negative" campaigning. "positive" campaigning is when a candidate talks about his or herself, and "negative" campaigning is when the candidate criticizes his or her opponent. personally, i think "negative" campaigning is important, just as important, if not as important, as positive campaigning. i want to hear candidates point out flaws in each other's proposals and explain to me why they think their idea is better. that's constructive, and it's necessary if we are going to have a substantive campaign on the issues.
the problem is that no distinction is made between constructive negative campaigning (e.g. "my foreign policy is better than his foreign policy because of X, Y and Z") and nonconstructive negative campaigning (e.g. "my opponent is a devil worshipper"). when people hear the term "negative campaigning" or are asked about when politicians "attack their opponents", nonconstructive negative campaigning is what comes to mind. they don't think about the constructive version because words like "attack" and "negative" are, well, negative. when they hear such words they associate it with things they don't like--negative things--which is a narrower category than what "negative campaigning" technically means.
it has become common wisdom that the public gets turned off by "going negative". but i wouldn't be surprised if the only time the question is polled they use a term like "negative". calling something "negative" from the get-go is sure to bias the results. i wonder if anyone has polled it by using examples rather than labels. if they did, i suspect the results would be a little different. my sense is that people want their candidates to "be assertive", "point out flaws in their opponent's positions" and to "take on the opposition." if you put it in those terms this negative campaigning stuff doesn't sound all that negative after all.
