You have to marvel at America’s economy. Not long ago it was widely thought to be on the brink of recession. Instead, it ended 2023 nearly 3 per cent larger than 12 months earlier, having enjoyed one of the boomier years of the century so far. And it continues to defy expectations.

At the start of this year, economists had been forecasting annualised growth in the first quarter of 1 per cent; that prediction has since doubled. The labour market is in rude health, too. The unemployment rate has been below 4 per cent for 25 consecutive months, the longest such spell in more than 50 years. No wonder Uncle Sam is putting the rest of the world to shame. Since the end of 2019 the economy has grown by nearly 8 per cent in real terms, more than twice as fast as the eurozone’s and 10 times as quickly as Japan’s. Britain’s has barely grown at all.

The expansion is all the more striking considering the many things that could have killed it. As the Federal Reserve has fought inflation the economy has endured the sharpest rise in interest rates since Jimmy Carter was in the White House. The COVID-19 pandemic, an intensifying trade war with China and the fight against climate change have together reshaped supply chains, labour markets and consumer preferences. Wars in Ukraine and Gaza have aggravated geopolitical tensions and worsened the strains on the global trading system.

Can the remarkable strength persist? Threats to growth still hang over the economy. The longer interest rates stay high, for instance, the more damage they could do. Although inflation has fallen, it remains sticky above the Fed’s 2 per cent target, meaning that the Fed may be unable to fulfil investors’ hopes for interest-rate cuts starting in June. Geopolitical tensions, meanwhile, look likely to spur economic fragmentation. Yet the biggest threat of all stems from November’s presidential election. Neither President Joe Biden nor ex-president Donald Trump seems likely to nurture the economic expansion should they return to the White House. Instead, their plans would endanger it.

To understand this, consider the reasons for the economy’s extraordinary performance. A key plank was generous pandemic stimulus, which at 26 per cent of GDP was more than double the rich-world average. This largesse fuelled inflation but also ensured fast growth: consumers have yet to spend all the cash they received in “stimmy” cheques. Even as the COVID crisis passed, the government continued to borrow away. The underlying deficit over the past year was nearly 8 per cent of GDP. That supported demand even as rates went up.

Strong demand has been met by growing supply. America has 4 per cent more workers than it did at the end of 2019, thanks in part to rising workforce participation, but mainly owing to higher immigration. The foreign-born population is up by 4.4 million, a figure which may undercount those who arrived illegally. And the expanding workforce is being put to productive use. America’s flexible labour market has almost certainly made it easier for the economy to adapt fast to a changing world.

Other long-standing strengths have made America enviably placed to cope with geopolitical tumult. Its vast internal market encourages innovation and means it depends less on foreign trade than smaller rich economies do. Because the shale boom of the 2010s made America a net energy exporter, it has in aggregate benefited rather than suffered from the high energy prices that hit the wallets of Europeans.

The trouble is that each of the ingredients for growth can no longer be relied upon. It may be tempting for politicians to extrapolate from America’s recent success and juice the economy with further stimulus. But that is becoming unsustainable. Official forecasts show the US will this year spend more on debt interest than national defence. More borrowing risks building up financial perils in the future.