Why is the first lady so keen for Joe Biden to run again for the White House?

If there was anything more demeaning to Joe Biden than his performance at last week’s unusually early presidential debate, it was the lengths to which he and his wife and his spokespeople have subsequently gone to insist that there was essentially nothing to see.

A pitifully hyped Jill Biden led a day-after rally clearly designed to extinguish the impression from the debate, which left the equally unfortunate impression of a president tied to the apron strings of his ambitious wife. Since then, members of his team have mentioned mitigating factors, such as a heavy cold or jet lag, or just a bad day.

“I screwed up,” as Biden himself has acknowledged. He is said to have told Democrat governors that he plans to stop scheduling work events at 8pm so he can get more sleep. Perhaps he needs it: for his most recent gaffe, he announced on US radio that he was the “first Black woman to serve with a Black president”.

Expect more apologies and mitigating factors when his first televised post-debate interview – already being billed as a “make or break” moment – is broadcast on ABC tonight.

But it was his wife’s response to that disastrous first debate that floored despairing Democrats. They had made no secret of their hopes that Dr Jill, as the only person who could persuade him, would have a “tough but necessary” talk that would convince him to step down for his own and the party’s good. If there was any talk, however, it seems to have gone in the opposite direction, and she came across as the driving force.

At one time seen as a self-effacing college teacher, Jill Biden has seemed increasingly at home in the limelight – to the point of accompanying her stepson, Hunter Biden, to his trial for a firearms offence and posing in a white tuxedo dress for the latest issue of Vogue. The photoshoot may have been planned when it was assumed the debate would go well – certainly, the coverline “We will decide our future” now has a hollow ring. But the steely look she adopts in the cover portrait shows someone accustomed to being in charge, and with at least as much steel as Rosalynn Carter, the last “steel magnolia” in the White House.

With hindsight, she was probably not the person to ease her husband out of the race, at least not yet, even though the TV debate – the brainchild of the Biden team, not of his Republican opponent, Donald Trump – was far from the triumph they had banked on to allay concerns about Biden’s age and acuity.

While Biden seemed, for the most part, to comprehend the questions and while those protesting that he is not senile may well be right, not being senile is hardly a recommendation for another four years at the helm of the most powerful country in the world. A US president needs to appear presidential. He needs to have all his wits about him, and be seen to have all his wits about him – that’s what debates are all about. And he needs to carry conviction.

Lately, there has been too little of this. And however much his State of the Union address in March and a couple of rallies since the debate have shown a more convincing Biden, the central question remains. Can the United States afford to elect for another four years someone who has such glaring off days, who can seem unaware of his surroundings, who mangles his words, loses his thread, and succumbs to jet lag on his big night?

A majority of Americans may already have decided that the answer to that question is “No”. According to a new Wall Street Journal poll, 80 per cent of voters now believe Biden is too old to run for a second term. And why wouldn’t they – even if the alternative is someone with a hardly unblemished record after four years in the White House himself, and a string of criminal convictions? The first New York Times poll after last week’s debate showed a three-point shift toward Trump.

There are only really two arguments for sticking with Biden: that it is too late to change the candidate and that there is no one else. It is certainly very late, but that is an argument for acting now. And there is someone else – the superficially easy solution would be to draft in the vice-president, Kamala Harris. With Biden’s popularity now sinking in key swing, she is now polling higher than her boss.

A rhetorical question often used to cast doubt on Harris’s presidential potential, however, highlights the problem: what if she had been president during the Cuban missile crisis? As things stand, though, the same question could now be asked about Joe Biden.

With Jill Biden having apparently rejected pleas to persuade her husband to step down, and with Democratic governors now rallying to his cause, it has been left to a few outlier Democrats and the media, led by the New York Times, to make the case for a replacement candidate. In the end, though, it is probably donors and poll ratings that will now decide.

The lack of any obvious Plan B is really no argument either. After all, cometh the hour, cometh the man or woman. With some plausible candidates reportedly shying away or deemed unelectable, the spotlight is currently trained on Michelle Obama, though with no evidence at all as to whether the former first lady might be willing to carry the torch for the Democrats, let alone spend another stint in the White House – or how her nomination might be engineered. For now, there’s succour in the fact that polling shows her to be the only Democrat who could comfortably beat Trump, with an 11-point lead.

It might be added that the testing provided by a long presidential campaign can be seen as a desirable, even necessary, preparation for the job. Someone whose profile looks perfect, even someone with first-hand experience of the White House, may not, in the event, possess the qualities to be even an adequate president.

Whichever way the Biden question is resolved, however, one apparently small, but necessary change, could perhaps come out of it in the longer term: an amendment to the Constitution setting a maximum age for the president.

The US Constitution is an admirable document in many ways. It steered the country more or less successfully through the mess of the 2000 election, when it turned out there was no means of conducting an accurate recount in Florida. You can quibble with the electoral college system, which allows someone to win the presidency while losing the popular vote, and with the scope of presidential immunity, as interpreted by the Supreme Court this week, which will doubtless keep lawyers very gainfully employed for a long time to come.

And amending the Constitution is not designed to be easy: there have been only 27 amendments in more than 200 years. However, in the light of Biden’s apparent determination to stand for re-election at the age of 82, and the entirely justified questions that have arisen about his fitness for office, the next Congress might just be inclined to have a go. After all, it was the spectacle of an ailing Franklin Roosevelt during the concluding months of the Second World War that led to the 22nd Amendment to be added in 1951, setting the two-term limit for future presidents.

Instituting a maximum age at which someone can run for presidential office might be seen by some as a restriction on personal freedom, but there is an argument that it is now rational and necessary. There is, after all, a minimum age for being elected president, which is 35 – why no maximum? There would be no point in introducing a compulsory retirement age, as an electoral mandate would surely pre-empt any retirement age. A maximum age on the date of inauguration, however – how about 75? – might have a better chance of being enshrined as the 28th Amendment, while memories of the Biden embarrassment, whatever the outcome, remain fresh.

The current state of affairs is good for no one. It is endlessly demeaning for Biden himself and his family to have his mental capacity repeatedly questioned in this way. It is a liability for the Democratic Party, which could lose the presidency on this issue alone, long before any vote has been cast.

Above all, though, it is bad for the United States, whose international image and authority – already perhaps more contested than at any time in recent memory – cannot but be impaired by a president who seems increasingly incapable of cutting a credible figure in a TV studio in his own country, let alone on the global stage.

Some still pin their hopes on the first lady as the one person who could prevail on her husband to see sense, at which point the whole dynamic of the race would be instantly transformed. To the disappointment, even despair of many Democrats, however, there is little sign of that yet.