I guess the focus groups must have responded well to Jack Layton's post-budget comments comparing paying down the national debt to paying down your mortgage, because he's used the same quote again and again ever since.

You'd think the NDP leader would be a little sensitive on the subject, having embarrassed himself in those same post-budget comments by attacking the government for proposing to spend $200-billion on paying down the debt -- an apparent reference to its goal of reducing the debt-to-GDP ratio to 25% over ten years. It was a colossal gaffe: Layton's numbers assume the target would be reached this year, or at any rate entirely by reducing the debt in absolute terms, with no help from economic growth whatever.

No matter. The NDP plainly thinks it sees an opening in what it calls the Liberals' excessively stringent policy of debt reduction. (Stringent? Policy? It's a residual. They pay down whatever's left over after their traditional multi-billion-dollar year-end blowout. If that's stringent, I'd hate to see what lax looked like. Actually, I have.) And it is, admittedly, a great quote, making debt reduction seem not only wrong-headed, but insane.

Here's Layton, on budget day: "It's like paying down the mortgage faster when you've got a leaky roof, a sick grandmother and your child is trying to go to university."

And again, two weeks later, in an interview with the Edmonton Journal: "When it comes to the Martin budget, you can imagine an emergency family meeting. The roof is leaking, granny is sick and needs medicine, there's no money for the oldest daughter's university books and the fridge is empty. And the father says: We should pay off the mortgage."

And again, last Sunday, on CTV: "It's a little like if you were faced with the proposition to reduce your mortgage faster than the bank said you had to but you had a sick grandmother, a child who wanted some education and your roof was leaking and the family said, no, we're not going to take care of those things, we're going to pay the mortgage down faster."

Okay, okay, we get the message: Mortgage, grandmother, university, roof. What is wrong with this analogy? Let me count the ways.

A mortage is backed by a hard asset -- typically, your house -- which can if necessary be sold off to repay the bank. Indeed, the mortgage is usually for less than the value of the house: the difference is the equity you hold. But the national debt is not gross debt, like your mortgage: it's the net debt, subtracting the government's assets from its debts. Since the latter far exceed the former, the federal government in fact has no equity: it has a negative net worth.

Much of that debt was incurred, as the foregoing implies, not to invest in assets that would produce a continuing stream of returns, either to society or the government, but rather to finance current consumption, notably for income-maintenance programs like pensions and EI. So it's less like a mortgage, more like a credit card -- with no pre-set limit!

Unlike a mortage, moreover, which you probably negotiated with your local bank manager, and with whom you could plead your case if you were having trouble making the payments, the national debt is owned by thousands of notoriously skittish investors, many of them overseas, who would be quick to dump their holdings at the first whiff of trouble. For example, if the leader of a party who could not tell the difference between the numerator and the denominator in a debt-to-GDP ratio were ever to get anywhere near the levers of power, it's safe to say Canadian debt would be marked down to move.

Finally, it's all very well to talk about Grandma's operation and Sis with her dreams of college, but there's another member of the household Jack's neglecting -- someone, in fact, who looks very much like him: a fifty-something baby-boomer. Soon, sooner than he wants to believe, dear old Dad is going to be hitting retirement age. And who's going to look after the family then?

In sum, the correct comparison is not with the overly prudent mortgage-holder of Layton's homely analogy, but rather an aging roué who racked up huge debts over many years on some dodgy foreign credit card he got in the mail, and now hopes, just before setting off for Florida in the RV, to stick his kids with the bill -- if necessary by holding Grandma to ransom.

How huge are those debts? The national debt currently runs to about half a trillion dollars, or nearly three times as much as the federal government takes in every year in revenues. Just to service that debt consumes 20 cents of every dollar the government receives . (It was 36 cents at its peak.) So it's as if a household with, say, $60,000 in annual income were having to cough up $1000 a month just to pay the interest on its credit cards. No wonder Grandma's sick.

But redemption is ever at hand. If feckless Pops makes a start on straightening out his chaotic finances now, perhaps Sis will be able to support the family once she's in the workforce. If she needs to, she could even borrow more to tide them over any short-term troubles. But only because Pops had the foresight to pay down some debt now, and thus cut her some slack with the banks.

As for that leaky roof, why do you think they call it "saving for a rainy day?"