It’s a battle that 77-year-old Senator Bernie Sanders, Biden’s main rival for the Democratic nomination in next year’s election, must also face.

Sanders has admitted that “age is a factor” but has argued that a candidate should not be judged on their colour, gender or how old he or she is.

Even Trump himself seems to believe the oldest two candidates will triumph against the younger ones in the Democratic field, tweeting: “I believe it will be crazy Bernie Sanders vs. sleepy Joe Biden as the two finalists.”

It helps Biden and Sanders that early polls show them not only leading all the other 18 contenders in the race but also that either of them is capable of defeating Trump.

Indeed, they were the only two Democratic hopefuls that a majority of party loyalists said had at least some chance of winning the 2020 election.

The polls suggest Biden has an early lead over Sanders and both have high approval ratings among Democrats.

In an ordinary election, this might suggest the party would be anxious to unite behind one or other of the septuagenarians but this is no ordinary election because Trump’s impact on the Democratic party, as on the nation itself, has been profound.

The further right the president goes, the further left the Democratic party tends to veer. It may be a smart move by Trump, who likes to paint his rivals as far-left socialists. But for the Democratic party it’s sparking the kind of infighting that could have disastrous consequences for their election chances.

Enter Biden, who is already casting himself as the candidate, seasoned by life and politics, who can unite the party and bring down Trump. He is offering experience and a character forged by tragedy that has included the deaths in a car accident of his first wife and infant daughter in 1972 and his son Beau, who died of cancer in 2015.

Certainly, Biden is someone who can rally the crowd and bring out the Democratic vote. But so can Sanders, as he proved during his 2016 bid for the party’s nomination when he almost toppled Hillary Clinton.

In many ways, their “old guard” battle for the nomination mirrors their own rivalry in the battle for the two wings of their party — Sanders is wooing the younger left-leaning members, while Biden is seeking the support of the older middle-of-the road wing.

Biden is buoyed by the fact that in a similar battle to become Democratic Speaker of the House of Representatives after the 2018 elections, 79-year-old Nancy Pelosi was able to defeat a battle for generational change by the younger, newly elected members.

But many in the younger wing are determined to fight harder for a like-minded, left-leaning candidate who will embody the more progressive policies of their party. “Joe Biden stands in near complete opposition to where the centre of energy is in the Democratic party today,” Alexandra Rojas of the Justice Democrats progressive group said after Biden entered the race on April 25.

“Democrats are increasingly uniting around progressive populist policies like Medicare [healthcare] for All, a Green New Deal [on climate change], free college, rejecting corporate money and ending mass incarceration and deportation.”

Sanders is sympathetic to their demands but Biden is more sceptical. “The fact of the matter is, the vast majority of the members of the Democratic party are still basically liberal to moderate Democrats in the traditional sense,” Biden said earlier this month.

Show me the really left, left-wingers who beat a Republican. So the idea that the Democratic Party has kind of stood on its head, I don’t get.

While Trump tries to cast Sanders as a “crazy leftist,” he cannot define the more moderate Biden in those terms — so a Biden candidacy would upend one of the main prongs of Trump’s re-election strategy.

Some Trump advisers see the former vice president, with his mainstream blue-collar appeal, as a tough opponent in the three states that carried Trump to his improbable victory in 2016 — Michigan, Wisconsin and Pennsylvania.

Losing any of those states in 2020 would make Trump’s path to re-election more problematic.

“They think they’re in trouble there and they think he’s a real threat,” said one Trump adviser, who spoke on condition of anonymity.

Biden, and less so Sanders, could also virtually wipe out Trump’s advantage with older, white male voters. Polls show that while Trump leads with such voters at 41% to 38% for Biden, his lead is only within the margin of error of most polls. Biden, still smarting from accusations of touching women inappropriately, also manages to lead Trump 45% to 28% among females.

Biden is very much an old-style Democrat in the Irish-American mould, who believes that working class, union Democrats and moderates in the suburbs, whom Hillary Clinton often ignored to her cost, remain the backbone of the party.

Biden also wants to make the case that he is the most electable candidate in key states such as Ohio, Michigan and Pennsylvania, where embracing liberal positions could hurt him.

But the younger candidates among the 20-strong Democratic group will also be seeking to cast both Sanders and Biden as too old for the 2020 battle.

“It’s time for a new generation of American leadership,” declared the youngest hopeful in the race, 37-year-old Pete Buttigieg, Biden and Sanders are the only candidates in the Democratic race who are in their 70s. The other 18 are mostly in their 60s, 50s or 40s.

But with age, comes experience. Biden and Sanders have over 60 years congressional experience between them and are also the only candidates who have been previously tested in a presidential race — in Sanders’ case once, in 2016; and in Biden’s case twice, in 1988 and 2008.

In the ordinary scheme of things, it might seem if a candidate fails once and certainly if he or she fails twice that another bid is unlikely to be dramatically different. But these are not ordinary times in American politics.

This is Trump’s America, where politics has become coarser, harsher, sometimes almost pitiless in the pursuit of power and where demagoguery rather than decency rules.

So, in this America, a candidate moulded in the old-school political values of tolerance and collegiality may be the antidote the country craves — and of the two men, Biden comes across as more the old style “political gentleman” than the more abrasive Sanders.

Then again, it was Biden who said he was spoiling for a fight after Trump used profanities in a tape before he entered the race that referred to lewd behaviour with women.

“If we were in high school, I’d take him behind the gym and beat the hell out of him,” Biden declared.

We can only hope, however, it doesn’t come to that in the 2020 race.