THIS week, Democrats are meeting in Philadelphia to nominate Hillary Clinton as their candidate for the presidency. In between the speeches, party business, musical acts and other convention agenda items, the party has been airing short, funny little videos, starring former members of Barack Obama's economic team. Here's one that ran last night, featuring Gene Sperling, who most recently served as the director of Mr Obama's National Economic Council:

Comically, it's not bad. Politically, it maybe works, or maybe it doesn't; who knows. But economically this is a little funny. Donald Trump has all sorts of disastrous economic ideas, such as they are. But would massive, permanent tax cuts for the very rich, and the debt they generate, "tank the economy", as Mr Sperling says?

There might be some small, positive supply-side effect to the tax cuts, but the history of the last few decades suggests we shouldn't expect anything massive; indeed, the effect of the tax cuts on incentives would probably be lost amid other influences on the economy. On the demand side, permanent tax cuts for the very rich would be expected to have a multiplier far smaller than tax cuts for poorer workers or direct spending, but even so, new deficits of several percentage points of GDP could not help but raise demand (assuming the Fed did not react to the plan by raising interest rates sharply). It would be an inefficient stimulus, but it would be a stimulus, and given that the American economy is still close to the zero lower bound and is probably still operating with a fair amount of economic slack, such a stimulus should increase growth in real output for a year or two at least. In the short run, the economy would not crash (not as a result of Mr Trump's tax cuts, anyway).

What about the longer run? Well, $34 trillion in new debt over the next two decades would be a lot, without question. It probably wouldn't be a catastrophic amount, however. The Congressional Budget Office estimates that American GDP will be around $40 trillion in two decades. That suggests that Mr Trump would raise the ratio of gross debt to GDP by about 80 percentage points relative to what it would otherwise be. On top of the growth CBO already projects for American government debt over the next two decades, that would push up gross debt to GDP to about 220%.

That's a lot! On the other hand, it implies that America would have a ratio of debt to GDP two decades from now that is 30 percentage points less than Japan's government debt is right now. How probable is it that America follows the Japanese path, though, rather than one in which bond markets balk at American bonds and interest rates rise, pushing the country into a Greece-like debt spiral?

In producing its debt estimates, the CBO assumes that the 10-year real interest rate will be roughly 2% over the horizon considered. Correspondingly, it projects that net interest costs will rise steadily over the period, accounting for a growing share of annual deficits and of cumulative debt. How realistic is that, though? At present, the real 10-year interest rate is roughly zero. Looking around the world, Japanese dynamics look like the norm rather than the exception. And that suggests that debt will grow less than projected over the next two decades, because interest costs will be lower. On the other hand, if massive stimulus succeeds in pushing America out of the low-rate trap, it should also raise nominal GDP growth rates above what is forecast. In that case, interest costs might be as high as CBO projects, but growth should be higher, holding down the debt-to-GDP ratio.

Think also about the matter of countercyclical policy. If America remains stuck in the low-rate trap, then the ratio of gross debt to GDP might rise to 200% or more, but the net ratio, or the amount of debt held by the public, might rise by far less. That is because the Fed will find itself needing to reactivate quantitative easing in future in order to respond to shocks, since it will not have much room to cut rates. Japan's gross debt is eye-popping, but its net debt is considerably smaller, at just 130% of GDP: less than the level in Greece and only a little higher than in Italy and Portugal. Mr Sperling jokes that you can't print money to finance a massive debt. And yet Japan seems determined to prove that, actually, you can. Either Mr Trump's plan is so massive that it will sate the world's appetite for safe assets, boost demand, and launch America out of its low-rate trap, or it will be a macroeconomic nothing-burger, which boosts the fortunes of the rich and raises debt-to-GDP ratios but which has a surprisingly small effect on growth, inflation and interest rates.

The point here is not that Mr Trump's plans are sensible. To be clear: they are ridiculous. It is to illustrate an uncomfortable truth, though, which is that the world economy right now is also a little ridiculous. Government bonds are not supposed to be doing what they're doing. Countries are not supposed to be able to borrow like Japan is borrowing. So while playing to people's gut instinct that fiscal sobriety is good is probably good politics, it might also be an obstacle to political efforts to respond in a sensible way to the weirdness of the economic moment: by making a positive case for deficit-financed public investment and for a change in central bank targets to something better suited to the moment, for example. Dangerous demagogues like Mr Trump thrive in conditions like those now afflicting the global economy. If the adults in the room remain unwilling to take macroeconomic challenges seriously, Mr Trump's debts will be the least of our worries.