Never mind leading the free world, if Donald Trump were your ageing father, when would you take away his car keys?

Donald Trump’s cognitive skills are amazing. So amazing! So great! So much better than any other dumb presidential contender you could mention, at least according to Trump himself, who bragged once again last week of how he had repeatedly aced what he calls “a very hard test for a lot of people”. (It’s thought he means a screening tool for mild cognitive impairment in elderly people.)

Sure, the 79-year-old leader of the free world recently interrupted a cabinet meeting in the middle of a war to ramble on at length about a conversation he supposedly had with the head of the Sharpie pen company over supplying bespoke presidential felt-tips, of which the firm said it could find no record. And made a baffling joke about Pearl Harbor during a press conference in front of an alarmed-looking Japanese prime minister. And called the strait of Hormuz the “strait of Trump”, before adding that that was absolutely deliberate because “there are no accidents with me”. But anyway, to be clear, his mental state is great. The greatest!

Just suppose, however, that it wasn’t. Imagine, purely for the sake of argument, that the 61% of Americans (according to Reuters-Ipsos) who think their president has become more erratic with age and the 56% who don’t think he has the mental sharpness now to deal with challenges (according to recent polling for the Washington Post) were not wrong. Suppose that, much as they did with an octogenarian Joe Biden, millions of Americans had sensed something through their TV screens that genuinely did affect their president’s capacity to send thousands of young soldiers to their potential deaths in the Middle East, whether or not that something amounted to a clinical diagnosis.

Imagine they were right to suspect that the lives of countless people around the world rested in the hands of someone whose judgment might not be entirely up to this – including the 45 million estimated to be at risk of acute hunger if farmers can’t get enough fertiliser, a crucial byproduct of a now badly disrupted Gulf gas industry, to grow food. What would it take, hypothetically, for the system to challenge an elected president’s will?

It’s strange that this has become a subject seemingly too delicate to discuss in public, given what is at stake. Though the US has checks and balances to stop a president veering off piste, none seem iron-clad. The ultimate backstop is the requirement to seek Congress’s approval before declaring war, which could yet end this conflict and prevent others in future, perhaps over Greenland or Cuba.

But Trump has got as far as readying almost 10,000 troops for deployment to the Middle East in preparation for an invasion without it, and numbers that big can create their own momentum in conflict (though Trump may simply be bluffing about taking Kharg Island, the hub of Iran’s oil industry, the Iranians may not know that). Ripples of alarm are finally breaking the Republican party’s surface, after a classified briefing for lawmakers last week, after which South Carolina representative Nancy Mace emerged warning that the military objectives they were given weren’t the ones given to voters. But for now, Wall Street still seems a more powerful restraining influence than Washington, with traders reportedly devising their own formula for how far markets have to fall before Trump gets cold feet.

If all else fails, under the 25th amendment, a president can have their powers suspended if the vice-president and a majority of the cabinet agree they aren’t fit to serve. But in practice, that tends only ever to be invoked briefly with the president’s consent, for example, as in George W Bush’s case when he underwent surgery under anaesthetic. Even the regular medicals that US presidents undergo don’t entirely guarantee transparency, with the true extent of Biden’s frailty becoming clear only after he abandoned the idea of running for a second term. Though Americans had grown suspicious of his tendency to forget names or look confused in public, what they didn’t know until the journalists Jake Tapper and Alex Thompson published their 2025 book, Original Sin, was that his doctors had warned he might need to start using a wheelchair.

Similarly, the public didn’t know until after his death that President John F Kennedy’s doctor had him on a cocktail of amphetamines and steroids during the Cuban missile crisis, just as Britons didn’t know at the time that Winston Churchill was a heavy enough drinker for the then US president, Franklin D Roosevelt, to mention it in cabinet, or that Harold Wilson most likely had the beginnings of dementia during his final days in Downing Street. In practice, constitutional safeguards are only as strong as the resolve of a leader’s inner circle – people often devoted to keeping them in power at all costs – to expose the boss publicly at his or her most vulnerable.

Why would anyone hide the truth about a physically or mentally faltering leader, given the potential consequences? Fear is the obvious answer: of reprisal, loss of influence, or even public panic should the truth come out in the middle of a crisis. But a less obvious answer is the ferocious loyalty and protectiveness, even love, that longtime service in political trenches breeds.

If you are reading this as the son or daughter of elderly parents whose memories have begun to falter, you will know how long it often takes from the first uneasy gut feeling that something has changed to what might ultimately lead to a conclusive medical diagnosis – and how many sleepless nights lie in between. Should they really be driving still, or have they become a danger to everyone on the road? Is it safe for them to manage their own money, or is it time to have an awkward conversation about powers of attorney? The fear of barging in too early, causing a hurt and outraged octogenarian to dig their heels in, clashes with the guilt of knowing that it will be your fault if they run someone over while you’re still agonising over taking away the car keys.

But it’s precisely to override such emotional dilemmas that, in the case of political leaders, constitutional safeguards exist. For without them, we’re all potentially just passengers in some superpower’s speeding truck: watching helplessly from the back seat as the driver weaves all over the road, and wondering just how close we have to get to crashing before someone speaks up.

• Gaby Hinsliff is a Guardian columnist

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