Hamish McRae: Donald Trump is playing with fire that could get us all burned

But that estimate may be too low, and S&P acknowledges that there are all sorts of indirect effects beyond the immediate losses. Some of these are hard to estimate.

For example, you can calculate the losses for the local community if people cannot visit a national park, but it is hard to put a number on the costs if a string of initial public offerings are delayed or pulled because the SEC cannot process them.

There is a further twist. The shutdown hits revenues. Americans still have to file their tax returns by April 15 and pay their taxes, but the US Internal Revenue Service has laid off the workers that collect back taxes, so there will be delays in tax collection. However, the administration has required that tax refunds are still sent out, so the overall deficit will rise, at least temporarily.

Add everything together, the direct and indirect costs, and what do you get? Well, the Bureau of Economic Analysis looked at the effect of the partial 16-day shutdown in October 2013, and though it acknowledged that these couldn’t be quantified, it did estimate that the direct effects cut fourth-quarter GDP by 0.3 percentage points. Inevitably the indirect costs will have been larger, but since the BEA is now itself shut (it is part of the Commerce Department), it will be a while before we get any official feeling for the magnitude of this one.

Indeed there may be delays in fourth-quarter and full-year GDP estimates, which are supposed to be out on January 30. Thanks to the shutdown the US economy is pretty much flying blind.

In the absence of data, you have to rely on intuition. My instincts are that this hit will end up knocking at least one percentage point off first-quarter GDP, even if resolved in the next couple of weeks.

That may seem alarmist. JPMorgan has cut its estimate of first-quarter growth, but only from 2.25% to 2%. (In the US, quarterly estimates are calculated as an annual rate.) But CNN reports an executive at Fitch Ratings saying that the US may lose its AAA credit rating if this drags on. If that happens then the cost of funding the US government would inevitably climb, and that, in turn, would push up borrowing costs in the US generally.

It is a tricky moment for the US economy. It has been running full bore, with unemployment the lowest since the mid-Seventies and job-creation still racing on. The tax cuts have given it a further lift. It now has to ease off, calm down and slow to a long-term sustainable rate. What you don’t want at this stage is an unquantifiable and unnecessary shock. In social terms this is miserable, because it hits good, hard-working people who have done nothing wrong.

In economic terms it is troubling not only for the loss of output, but because it makes a difficult transition harder still.

This is the world’s biggest economy and we all benefit from its vigour and innovation. We don’t want any mistakes.