Imagine it's Wednesday, November 4, 2020.

Biden Beats Trump! Former vice-president Joe Biden has become the oldest-ever President-elect of the United States, following a resounding election victory over incumbent President Donald Trump. Biden, who turns 78 later this month, inherits an economy suffering its worst economic recession since the 1930s and a divided nation weary of illness and political scandal. President-elect Biden says it's time to heal and unite America.

The possibility of such a story appearing in just under seven months from now became significantly greater this week with the decision of Biden's last rival for the Democratic nomination, Senator Bernie Sanders, to drop out of the contest.

Biden's surprising dominance in the primaries once the voting moved into larger, more diverse states in late February, was not only the key to defeating Sanders, it spells trouble for Mr Trump.

Biden's strength as a candidate was not down to his performance on the campaign trail or in debates, which was patchy at best.

He wasn't new and exciting like Kamala Harris or Pete Buttigieg. He wasn't promising to fundamentally rebalance the economy in favour of ordinary Americans like Bernie Sanders or Elizabeth Warren. And Biden certainly didn't have half a billion dollars to splash on the campaign like Michael Bloomberg.

Pete Buttigieg, Bernie Sanders, Joe Biden and Senator Elizabeth Warren in February. Two months later Biden has won the nomination. ( Reuters: Brian Snyder )

In the largest and most diverse primary field in history, Joe Biden was an old, white, male politician, with more than three-and-a-half decades in the US Senate, eight more as vice-president and two failed White House campaigns behind him.

Biden topped most national polls throughout 2019 but struggled to raise much money.

He crashed to fourth place in first-to-vote Iowa in early February, then placed a dismal fifth in New Hampshire the following week. His campaign looked gone for all money.

So what got him over the line?

But as it began to sink in to Democrats they may be about to nominate either a 37-year-old gay man (Buttigieg) or a 78-year-old socialist (Sanders) to take on Trump in November, Biden got a second look.

The former VP's ability to appeal — not just to older white voters, but black voters as well — proved decisive.

Joe Biden — with Barack Obama following the 2012 election victory — has reassembled Obama's electoral base. ( Reuters: Larry Downing )

He swept to victory in the states both Sanders and Trump performed well in four years ago.

Which brings us to another of Biden's strengths — not being Hillary Clinton.

Not being Hillary Clinton helped propel the relatively inexperienced senator, Barack Obama, to the Democratic nomination and then the White House in 2008, and it may well do the same thing for Biden in 2020.

He has all but reassembled Obama's electoral base, with the exception of younger voters, who have continued to support Sanders.

Without Hillary Clinton to run against, Bernie Sanders could not broaden his appeal.

His decision to leave the campaign now rather than risk further dividing the Democratic Party is a gift to Biden, and one Sanders denied Clinton in 2016.

And without Hillary Clinton to run against, Trump will struggle to hold onto older, white, working-class voters, whose anger he felt in 2016 but whose lives he has failed to improve four years later.

What about the incumbency advantage?

It is true that historically, incumbent presidents tend to be re-elected.

In the past century, only Herbert Hoover, Jimmy Carter and George H W Bush failed to hold on to the White House if they stood a second time.

But in the climate of a global pandemic and economic dislocation, many of the traditional advantages of the incumbent will be lost to Trump.

Instead like Hoover, Carter and Bush, a weak economy will make re-election very difficult, an inept response to a deadly virus could spell his doom.

Biden, on the other hand, may now possess the familiarity and steadiness that voters crave. They certainly did in the primaries, and may do so again in November.

He promises what Warren Harding once called "a return to normalcy".

After beating 28 other Democrats, Joe Biden will now face his final opponent — President Donald Trump. ( AP: Evan Vucci )

Trump is currently enjoying some of the highest approval ratings of his presidency, although he remains the only president never to reach 50 per cent support in the Gallup Poll.

In the average of national polls, Biden leads Trump by 49.7 per cent to 43.3 per cent. A consistent 6.4 per cent margin bodes well for Biden. Obama won in 2008 with a 7.3 per cent buffer over John McCain and again in 2012 with a 3.9 per cent lead over Mitt Romney.

However, in 2016, Hillary Clinton lost to Donald Trump despite winning the popular vote by 2.1 per cent, so while Biden is in the box seat, he can't take anything for granted.

As the headline-writers found out in 1948 when they pre-emptively proclaimed "Dewey Beats Truman!", anything can happen.