Clinton doesn't agree with McCain's idea. She'll do it only "if we could make up the lost revenues from the Highway Trust Fund." But we can't make up the lost revenues from the Highway Trust Fund, so she won't do it. And that's the right answer, but she's successfully confused most of the audience into thinking she does favor the holiday.
Matt thinks this duplicity is "pretty neat". But does it really help the Democrats to pretend to support stupid Republican ideas? [Once again: Bad Means Have Consequences.] I would have thought a better long-term political strategy would be to try to convince the public that those are actually bad ideas. But it's kind of hard to do that when you're too spineless to publicly admit that you disagree with them.
Imagine if Clinton were instead to vigorously denounce McCain as "economically illiterate" for proposing such a stupid policy, and thus "incompetent to be president". Such strong words might lead to further probing and investigation as to whether the charges were justified; economic experts would be called in to offer their opinion, and to explain in plain terms precisely why subsidizing gas is an idiotic policy (and perhaps propose more efficient forms of financial aid instead).
It's not impossible to change public opinion, especially when you have the truth on your side. I mean, to an uneducated layperson, the idea of printing money and making everyone a millionaire overnight probably sounds even more tempting than cheap gas. But I assume if a politician tried pandering to this ignorance, they would pretty soon be called on it, and ridiculed mercilessly. Why doesn't the same happen here?