“Why don’t you balance the budget at 24 percent [of GDP] instead of 19 percent?” I asked.

“I think it would do damage to the economy,” Rep. Paul Ryan replied.

This simple exchange from a conversation I had with Ryan in his office last October captures the uber-debate the country needs to have. That is, once we get done dissecting the deceptions, hypocrisies and regressive priorities in the Wisconsin Republican’s latest blueprint.

For starters, Ryan’s assumption that higher levels of spending and taxation would automatically hurt the economy can’t be right. If it were, America would be a poorer country today than it was a hundred years ago, when the federal government taxed and spent less than 5 percent of gross domestic product. But we’re obviously vastly wealthier. That doesn’t mean there isn’t a limit beyond which higher taxes and spending would hurt. Just that we’re not close to that point. How can we be, when President Reagan ran government at 22 percent of GDP?

Federal spending has gone from recent norms of about 20 percent of GDP to 24 percent under President Obama, thanks to the lagging economy and spending on things like the stimulus and unemployment insurance. Ryan wants to get it back to 20 percent in the next few years and return taxes to their more recent norms of 19 percent, up from today’s recession-depleted 15 percent. (The nonpartisan Tax Policy Center said Tuesday that Ryan’s proposals would in fact fall dramatically short of 19 percent, but leave that aside for the moment.)

At first blush, Ryan’s plan sounds perfectly reasonable — until you remember that we’re about to retire 76 million baby boomers.

“I think the historic size [of government as a share of GDP] is about right, or smaller,” Ryan told me that day.

“But how can that be,” I asked, “when we’re doubling the number of seniors” on Social Security and Medicare, the biggest federal programs.

“Because we can’t keep doing everything for everybody in this country,” he said. “We should trim down a lot of other stuff we’re doing.”

This was unintentionally revealing. Ryan has sounded this theme before. “We are at a moment,” Ryan said in his State of the Union response in 2011, “where if government’s growth is left unchecked and unchallenged . . . we will transform our social safety net into a hammock, which lulls able-bodied people into lives of complacency and dependency.”

But what hammock is Ryan talking about? The only thing slated to grow the size of government in the years ahead is the retirement of the baby boomers. The doubling of the number of people eligible for Social Security and Medicare is what is driving all the increase in federal spending — along with the spiral in system-wide health costs, which afflicts Medicare along with all privately financed health care.

If those programs for seniors haven’t been a “hammock” until now, simply doubling the number of people eligible for them can’t turn them into a “hammock” tomorrow. When it comes to fiscal policy, we have an aging population challenge, and a health-cost challenge. We don’t have a “hammock” challenge.

The upshot? Ryan wants to use an aging America and the bogus but superficially appealing constraint of “historic levels of spending and taxation” to force massive reductions in the rest of government. That’s why the Center on Budget and Policy Priorities and others Tuesday were already calculating that Ryan’s new plan would basically zero out everything in government a few decades from now, save for Social Security, Medicare and defense.

The crucial thing to understand about Ryan is that he is not a fiscal conservative. He’s a small-government conservative. These are very different things. The fastest-growing federal program in Ryan’s new budget is interest on the debt, which nearly triples from $234 billion next year to $614 billion in 2022. He doesn’t even pretend to balance the budget until 2040, and then only under utterly dubious assumptions.

These are not the choices a fiscal conservative makes. A fiscal conservative pays for the government he wants. Ryan wants government smaller than the one Reagan led even as America ages, and he doesn’t want to pay for it. Instead he adds trillions in new debt and makes no bones about it.

“Why would you choose to have debt, as opposed to saying we’re going to pay our own way now” via higher taxes, I asked Ryan back in October. This even after spending cuts that most Republicans think won’t command public support. “Why is that a conservative value?”

“Because of growth,” he said. “What I don’t want to do is sacrifice an entire generation to having less than optimal potential growth because their parents didn’t fix this problem.”

Huh? A cynic would say Ryan would do anything to avoid acknowledging the need for higher taxes as the boomers age. The conservative darling just won’t go there. The less charitable assumption is that the congressman is confused.

There’s more to say on Ryan’s blueprint, and, in spite of my general hostility to his thinking, he deserves credit for putting his party’s head in the noose by calling (rightly, if imperfectly) for Medicare reform. But the first order of business is to expose Ryan’s overall plan for the misguided, misleading and unacceptable vision it represents.

Matt Miller, a co-host of public radio’s “Left, Right & Center,” writes a weekly online column for The Post. His e-mail address is mattino2@gmail.com.