Adam Boulton: As America declines into gerontocracy, who might step in to replace the president should the worst happen?

"Not worth a bucket of warm piss". John Nance Garner's words are the most famous assessment of the office of vice president of the United States.

"Cactus Jack" Garner's words have often been bowdlerised to "warm spit" but there's no denying he was in a position to know. He served Franklin Delano Roosevelt as VP for nine years up until Pearl Harbor in 1941.

Garner is alleged to have passed on his words of wisdom as an old man to Lyndon Baines Johnson when LBJ was considering the offer to be John F Kennedy's running mate in 1960.

Johnson took the job regardless and three years later when JFK was assassinated, he lived out the other cliche about the vice presidency. He was indeed "a heartbeat away" from becoming president of the United States (POTUS).

On the day he was shot, Kennedy had phoned Garner to wish him a happy 92nd birthday. Hours later, LBJ was sworn in as 36th president.

Many US vice presidents have been deeply and loudly frustrated while in office but, in spite of Garner's crude dismissal, the job is not worthless.

Of the 45 men who have been POTUS, a third of them, 15, previously served as vice president. Nine inherited the Oval Office when the incumbent died or resigned, including Johnson. The others, including Richard Nixon, George Bush senior and Joe Biden, were later elected president in their own right.

Beyond the common duty of representing the leader at important funerals, Britain's occasional deputy prime ministers should not be likened to the US vice presidents. Deputy prime minister is an honorific title with no constitutional role.

None of the people who have held it officially have got into Number 10, even though there has been a rapid turnover of prime ministers taking place around them.

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As it happens, Garner, who died just before his 99th birthday, was the longest-surviving US president or vice president until Jimmy Carter, who is due to reach his century this October.

Compared to today's frontrunners, Garner was a youthful 72 when he left public life. Carter was a mere 56 when he lost in 1980 to Ronald Reagan, who at 69 was then the oldest-ever president-elect.

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Aged 81 and 77 respectively, Biden and Donald Trump, the two people now vying to lead the United States until January 2029, are record-breakers.

They are set to be the oldest candidates ever to contest the presidency. As America declines into gerontocracy, there is exceptionally high interest in who might step in to replace them given the actuarial likelihood the worst - or something debilitating close to it - might happen.

Trump likes to play TV show-style games with his VP choice

This week on Fox News. Trump acknowledged the importance of picking a deputy "who is going to be a good president", before teasing his interviewer, Maria Bartiromo, that he wouldn't be making any announcement for "a little while".

As a veteran star of The Apprentice, Trump likes to play TV show-style games with his choice.

In his first successful bid for the White House, he didn't pick Mike Pence until 15 July 2016, ahead of the November election. To drum up excitement he could wait, as other nominees have done, until the Republican Convention which will take place in Milwaukee in mid-July.

One thing is certain: Trump will not pick Pence again, or anyone like him.

The right-wing governor of Indiana and former talk show host was widely derided as a faceless yes man when Trump put him on his ticket after some cosy chats.

But Pence turned out to have some backbone. The insurrectionists storming the US Capitol chanted "Hang Mike Pence!" and brought along a mock gallows. Pence later testified: "We'll tell the truth, we'll obey the law."

He failed to get much support and pulled his bid for the 2024 Republican nomination before the primary contests started in January.

Trump names two possibilities for first time

On the assumption that his various legal troubles will not prevent him from getting as far as the nomination, Trump used his interview to name two possible names for the first time.

Crucially both Senator Tim Scott and Governor Kristi Noem have already bowed down before Trump. They have not endorsed his big lie the 2020 election was stolen from him, but both claim it was not free and fair.

To Trump's delight, Noem of South Dakota sucked up further, declining to run for the nomination herself because "I could never beat him".

Scott did put his name forward but rushed to endorse Trump after he withdrew - further denting the chances of Nikki Haley, the only Trump challenger still standing, in next month's primary in their home state of South Carolina.

Of course Trump may not end up picking either of them but he does seem to be interested in broadening his appeal by considering running mates who are not white men like himself.

Other names speculated on include ethnic minority men such as Byron Donalds, a US congressman from Florida; former Trump cabinet member and surgeon Ben Carson; and 2024 Republican contender Vivek Ramaswamy.

The list of possible female candidates includes Governor Sarah Huckabee Sanders of Arkansas; Kari Lake from Arizona and Representative Elise Stefanik of New York.

Haley is still running for the nomination and ruled herself out on the campaign trail in New Hampshire last month declaring: "I don't want to be anybody's vice president. That is off the table." Otherwise, she would be best qualified to be his running mate.

Trump could also revert to type with Ohio senator and Hillbilly Elegy author J D Vance and Florida Governor Ron DeSantis heading the list of conventional choices.

The most prominent woman certain to figure in a Trump v Biden battle is Kamala Harris. Biden is committed to keeping the first woman vice president and vice president of colour on his ticket for re-election.

She is campaigning energetically on his behalf in South Carolina and has been energised by the Trump-packed US Supreme Court's decision overturning women's abortion rights.

Harris is not popular with some Democratic insiders who have urged Biden to drop her. Were Biden to become unavailable as a candidate, she is unlikely to be the first preference to run in his place.

The UK farce of deputy prime ministers

The machinations in the US are high politics compared to the White Hall farce of deputy prime ministers.

Only seven people have been given the title, most of them recently: Clement Attlee, Michael Heseltine, John Prescott, Nick Clegg, Dominic Raab, Therese Coffey and Oliver Dowden.

Labour's Attlee was the prime minister's wartime deputy but Winston Churchill advised the King to appoint someone else if he should die. Subsequent, mostly Conservative, prime ministers were similarly offhand with so-called "deputy prime ministers in all but name" such as Willie Whitelaw, Damian Green and David Lidington.

There was never any doubt that Gordon Brown was Tony Blair's real deputy, although Prescott had the title. There is still an important difference when Labour is in power. The party's deputy leader is now directly elected. The precedent is set that they will be appointed deputy prime minister. Angela Rayner may be about to find out that being deputy prime minister is worth more than an ice-cold bucket of her favourite "Venom" cocktail.