Andrew Sullivan makes a key point about Paul Ryan’s refusal during yesterday’s debate to explain how the Romney tax cuts will be paid for:

Ryan was hampered by an insurmountable problem on the impossible mathematics of the Romney budget. I think his inability to answer that question — how do you pay for it? — has to be the driving question now. The only way to afford it is to cut middle class deductions and middle class entitlements much more than Obama-Biden would. I’d love radical tax reform — but I’m not crazy enough to believe you can actually tackle the debt by cutting taxes and increasing defense spending and leaving Medicare basically alone (no ACA-style cost-controls) and only removing deductions for the very rich. It doesn’t add up. They know it. And when challenged — even by Fox News — he cannot provide the details.

The part of the debate Sullivan is talking about came after Martha Raddatz did what Jim Lehrer utterly refused to do — ask Ryan how his massive tax cuts, which disproportionately benefit the rich, would be paid for. Ryan kept repeating that this will b worked out between Romney/Ryan and Congress. Which is to say: Never mind those pesky details for the time being, we’ll work all that out after we’re elected.

It’s my sense that the Obama campaign is going to seize on this moment and hammer away hard at it in the days ahead. Here’s why: Dems believe one of the key reasons Romney’s debate performance was so successful is that he managed to dispel doubts about the lack of specificity in his plans, by projecting a general air of competence, authority and policy know-how. This, after the Obama team had successfully sowed doubts throughout the summer about Romney’s true policy intentions and the priorities driving them.

Dems hope Ryan’s evasions in the face of Raddatz’s questioning gives them a way to reopen that wound. So expect them to amplify the case that cutting taxes across the board in ways that hugely benefit the wealthy cannot be paid for without the middle class taking a big hit, whether in the form of ending loopholes the middle class enjoys or in the form of deep spending cuts to programs middle class Americans rely upon. And to make a larger case about whose interests Romney and Ryan would really prioritize and protect.