Mitt Romney deserves credit and criticism for his explanation of his tax plan at Wednesday night’s presidential debate.

He deserves credit for finally clarifying at least one detail: His number-one principle, he explained, is that he would not increase the deficit in order to lower taxes, and he stuck by his insistence that he would not reduce the amount of revenue that the federal government takes in. That seems to contradict Paul Ryan, his running mate, who over the weekend said that the Republicans’ highest priority is reducing tax rates. Ryan’s words reflected an ominous instinct to slash the government beyond recognition or balloon the debt instead of bargaining with Democrats to pay for the government Americans expect. Both mathematics and politics demanded that Romney repudiate that policy dead-end. And, to a certain extent, he did, essentially saying that, if his tax-plan math didn’t add up, he wouldn’t increase the deficit to pay for new tax cuts.

But he deserves criticism for waiting until now to explain even that. How is it that Americans didn’t already know that was his top priority? There are only a handful of weeks before election day, and for weeks before now economists and journalists have tried to run the numbers on the Romney tax plan. He proposed a 20 percent tax cut without explaining how he would pay for it. Then he remained strategically vague for so long that a simple statement of priority is remarkable, even though he still hasn’t explained which big, popular tax deductions he would eliminate, and for whom, and it sounds like his proposal would still require plenty of wishful thinking about revenue resulting from economic growth. It’s also fair to wonder how long Romney will stand by the latest version of his proposal.

But the discussion must also move beyond the mere question of whether Romney’s tax plan could possibly add up. Millions of boomers are set to demand ever-more-expensive services from Medicare, and the country still requires sensible federal investment in things such as education and infrastructure. How could a revenue-neutral, rather than a revenue-raising, tax plan possibly be sufficient? President Obama pressed a little bit on this Wednesday night, explaining that he favored increasing the federal take. But he did not press hard enough, nor did he explain why his own plan would be big enough to meet the challenges America faces.

That question, perhaps, is for another debate — with a more aggressive moderator.