Only in America, the land of the big gulp and the double scoop, is it possible to elect two presidents in one vote. Joe Biden will be the 46th President, providing he makes it to Inauguration Day, but Kamala Harris is likely to the 47th President well before the 2024 elections.

Some people worry that Donald Trump will have to be dragged from the White House in January. I’m more worried that Biden will have to be carried into it. He turns 78 later this month and will enter the White House older than the previous oldest incumbent, Ronald Reagan, was when he left it.

The average American male, whose veins admittedly are thick with Coke and Cheeto juice, pegs out at 78 years and 5 months. Biden will dodge that actuarial ambulance in May. Regardless, his mental and physical decline is obvious, and obviously accelerating.

He forgets what he is talking about. He fumbles his autocue lines. He shuffles and totters on short paces and weak legs. He gets a faraway look in his eyes and carries the viewer with him into an agony of silence, then returns angrily, emitting a plume of utter nonsense.

“I’ll lead an effective strategy to mobilise true international-of-a-depressure,” he bellowed at a rally in late October. On the same day, he called General Stanley McChrystal “Stanley McGeneral”. Last week, he introduced his granddaughter as his dead son, then corrected himself and confused her with another granddaughter.

Biden is an experienced moderate and a professional white man, that was why Obama chose him

It is hard to believe Biden is the best candidate the Democrats could find; harder still to believe that he will still be an 82-year-old President in 2024. Biden knows as much. He has already admitted that he will not run again: an unprecedented admission in a candidate. The voters who gave Biden a massive margin in the popular vote know this too. So do the Democratic strategists who boosted “Joe and Kamala” as a two-for-one.

Kamala Harris was essential to Biden’s victory just as Biden was essential to Obama’s victory in 2008. Biden is an experienced moderate and a professional white man. That was why Obama chose Biden, written off by the punditry, as his vice-presidential nominee in 2008: to pre-empt accusations of identity politics and radicalism.

Harris’s selection enraged the identity politicians and radicals of the Democratic left

This time around, Biden picked Harris, a less experienced moderate, for the opposite reasons. Harris is not a professional brown woman, but she is a professional woman who is brown-skinned. Though her identity cannot be separated from her politics, she has allowed its symbolism to speak for itself – not least because while Biden’s campaign could take the support of black voters for granted, it needed to win college-educated white women.

The irony is that Harris’s selection enraged the identity politicians and radicals of the Democratic left. Rather than praise her for breaking glass ceilings as San Francisco’s DA and California’s attorney general, they denounced her for having pursued tough sentencing for drug offenders, including in miscarriages of justice that sent black suspects to jail.

Strangely enough, the radical statements in the Democratic campaign came from Biden. It was Biden, not Harris, who endorsed gender reassignment for children in a televised town hall. It was Biden who, with his sudden opposition to outsourcing and his late-career promise to rebuild an industrial base over whose decimation he presided, turned Trumpy and defended the disenfranchised whites of the Rust Belt as a white Pennsylvanian.

This was smart footwork. The close results of these elections confirm that Trumpish populism is not an aberration. If “Trumpism” was always a doctrine in search of an author, then Biden’s pitch as Rust Belt protectionist was one of its more coherent expressions – and proof that Trump’s heresy has gone bipartisan.

That might not be the only heresy to cross party lines under the Biden-Harris administration. In October, Nancy Pelosi launched a bill to give Congress greater powers over “future presidents” in applying the 25th Amendment, which orders the Vice-President to take over in the case of the President’s death, resignation or “removal from office”.

The 25th Amendment was passed in 1967, to avoid the chaos that had followed the assassination of John F. Kennedy. It was deployed against a Republican President in 1974, when Richard Nixon passed the reins to Gerald Ford, and on three occasions when a Republican President was temporarily incapacitated due to colon surgery (Reagan, George W. Bush) and a colon inspection (George W. Bush).

The precedent exists for the Vice President to take over on health grounds. The constitutional mechanism, judging from Pelosi’s recent manoeuvres, is much on the minds of Harris’s fellow California Democrats. If and when Joe Biden becomes the first Democratic President to undergo deposition by the 25th Amendment – and I reckon it’s a matter of “when”, not “if” – then Kamala Harris will be America’s first female President.