Of course it's irresponsible for an elected official -- and not just any elected official, but a high ranking one, the Senate Majority Leader -- to make reckless, rumor-based allegations against a presidential candidate.
Is there any defense for Harry Reid? Only that it could be worse. I mean, he's not accusing Mitt Romney of a crime, or even necessarily something unethical -- by Romney's own standards, it would be more problematic for Romney to be paying any tax if he can legally avoid it, so by Romney standards there's nothing necessarily unethical about not paying taxes. On the other hand, while I suspect a lot of people agree with the Romney standard expressed that way, I'd also guess that most people would also assume that if Romney hasn't paid taxes he's probably cheating in some way. At any rate, this is no kind of healthy politics. The Majority Leader of the Senate is either amplifying unproven rumors or just making stuff up. Reid's free to do it, but the fair consequence is that he gets (or adds to) a reputation for being reckless and irresponsible.
And good for liberals who point that out.
On the other hand.
I disagree with one of those liberals, Steve Benen, who writes that "Romney had the high ground against a cheap shot." I mean, I see his point, but you know what? Romney had previously forfeited any claim to having any high ground on this one by violating the norm of presidential candidates releasing tax returns. I don't know that it's a particularly useful norm -- I don't really think it's likely that any presidential nominee will be undermined by anything in his or her tax returns, Romney included, and I'm really not interested in supposed conflicts of interest at that level. But that's irrelevant. It's a norm of modern presidential politics, and if you violate that, you have no high ground to begin with. Nor has Romney given any particularly convincing reason for why the norm should not apply generally or to him in particular; to the contrary, his campaign wound up justifying it was a lie about John Kerry's tax returns, a lie they repeated after it had been pointed out. So, yeah, invoke cliches about sowing and reaping, please.
That doesn't, in my view, excuse Reid. Just because someone is asking for a cheap shot doesn't make it okay to take a cheap shot! But the high ground is usually pretty vacant during presidential elections, and Romney certainly wasn't occupying it on this issue.