What's the difference between an attack ad and legitimate criticism?

The Edwards campaign has a new ad out, presenting Clinton contradicting herself during a(?) recent Democratic debate:


I came across this ad on Tennessee Guerilla Women, where the blogger accuses Edwards of `cut[ing] and past[ing]' a `scathing' and `nasty' ad, and implying that he has thereby, and unlike Hillary, `gon[e] negative'. There's also a link to a discussion on another blog that, from the excerpt, appears to be accusing Edwards of hypocrisy. I want to bracket the issue of hypocrisy, since it could be that Edwards is making a legitimate criticism that applies just as well to both himself and Clinton. Note that I also assume some criticism is legitimate. While we're rather fond of accusing politicians of `going negative', part of the process of campaigning is pointing out the failings and flaws of one's opponents. Indeed, going negative has gained such prominence that accusations of it are popular and often unfair and illegitimate attacks -- it's a way to shame one's opponent for revealing one's own flaws and failings.

Attack ads and legitimate criticisms lie on opposite ends of a spectrum. In the murky middle are ads that might be illegitimate and unfair attacks and might be legitimate criticisms. Which one is this -- attack ad, legitimate criticism, or in the murky middle? It's clear that the blogger I quoted in the last paragraph thinks it is clearly an attack ad.

But it's not so clear to me. First, the ad doesn't contain vague and emotionally-loaded descriptions of her policies and past actions. It's showing clips of her speaking. Next, we might worry about context -- perhaps these remarks were made in contexts that change their meaning. But they mostly seem to be pretty clear, so that means that it's at least not clearly an unfair attack. Third, we might worry about the fact that Clinton is speaking extemporaneously rather than carefully and precisely stating her views. She's speaking on her feet at a debate to explain her views in a general way rather than formulating policy in a precise way for implementation. So, again, there might be subtlety and nuance to her views that are being unfairly neglected. The portion of the ad on immigration might be especially worrying in this respect.

Let's think about that immigration portion a little more carefully. The ad wants to suggest that Clinton is being inconsistent. It seems to me as though she might be trying to avoid answering an obviously stupid question -- giving illegal immigrants driver's licenses isn't an issue that can be settled with a `yes' or `no' answer in thirty seconds. But then I wonder why she didn't just say that it's a stupid question, and far too complex of an issue to be settled so simplistically. Hence, she might be saying inconsistent things, and she might be caught off-guard with a spectacularly stupid question. It's not clear either way.

So, with respect to the immigration portion, it's not clear whether the ad is an unfair attack or a legitimate criticism. With respect to the other two reasons I can think of for calling an ad an attack ad rather than a legitimate criticism, it's at least not clearly an unfair attack, and probably a legitimate criticism. I can't think of any other reasons for calling an ad an attack ad rather than a legitimate criticism. So, considering these three reasons together, I conclude that the ad is in the murky middle, but very, very close to being a legitimate criticism. It's at least not clearly an attack ad.

Addendum: There's a fourth respect in which an ad could be an attack ad, and that's if it's promoting or appealing to some odious ideology (racism, sexism, heterosexism, ablism, xenophobia, and so on). This ad is clearly not doing that by any ordinary standard. It's not, as one commentator on the source post suggests, going after Clinton `on the basis that she's female'.