Mitt Romney’s defense of his tax plan continues to not pass the giggle test.

Here the GOP nominee is on “60 Minutes”:

[CBS News’s Scott] Pelley: You're asking the American people to hire you as president of the United States. They'd like to hear some specifics.

Romney: Well, I can tell them specifically what my policy looks like. I will not raise taxes on middle-income folks. I will not lower the share of taxes paid by high-income individuals. And I will make sure that we bring down rates, we limit deductions and exemptions so we can keep the progressivity in the code, and we encourage growth in jobs.

Pelley: And the devil's in the details, though. What are we talking about, the mortgage deduction, the charitable deduction?

Romney: The devil's in the details. The angel is in the policy, which is creating more jobs.

Pelley: You have heard the criticism, I'm sure, that your campaign can be vague about some things. And I wonder if this isn't precisely one of those things?

Romney: It's very much consistent with my experience as a governor which is, if you want to work together with people across the aisle, you lay out your principles and your policy, you work together with them but you don't hand them a complete document and say, “Here, take this or leave it.” Look, leadership is not a take-it-or-leave-it thing. We've seen too much of that in Washington.

The problem? Go back a few minutes in the interview, and Romney is saying that his tax rates “would be the current rates less 20 percent. So the top rate, for instance, would go from 35 [percent] to 28 [percent]. Middle rates would come down by 20 percent as well.”

So for tax rates, it seems, there’s nothing at all wrong with “take it or leave it.” Details on tax rates are absolutely appropriate; details on how to pay for the reduction, however, are apparently what “we’ve seen too much . . . in Washington.”

It’s deeply silly to claim that a specific proposal is the equivalent of saying “take it or leave it” — as Jonathan Chait notes, why can’t a proposal be a first offer, from which he’s prepared to negotiate?

Romney implies that Barack Obama’s presidency has featured these take-it-or-leave-it moments. This is flatly wrong, as anyone who remembers the debates about the stimulus, health care reform or anything else could tell us.

And perhaps Romney should recognize that presidents must submit a budget every year — budgets that (rightly) contain lots of details.

The interview serves as another reminder that Romney’s insistence on certain specifics — tax rates, for instance, and several specific tax treatments he insists on protecting against reform — mean that his other “principles” of revenue-neutrality and keeping middle-income taxpayers from tax increases just don’t add up, as the Tax Policy Center told us.

Unfortunately, CBS’s Pelley didn’t follow up with a question on why some details are apparently perfectly fine while others would be the dreaded “Washington” way of doing things. Nor did he follow up on the mathematical impossibility of Romney’s tax policies. As bad as it is for Romney to still be peddling this stuff six weeks after the TPC made it clear that his math doesn’t work, it’s even worse for a network correspondent to not be prepared to challenge him on it.

Bottom line: If Romney didn't have the details he insists on, his tax plan would be unremarkable and quite reasonable. It's the details, not the vagueness, that trip him up.