Perception is everything, and President Biden was perceived not to be up to a second term in the most important job in the world.
Age was catching up with him physically and mentally. He looks and sounds like a frail old man.
Republican nominee Donald Trump had become odds on favourite to win. His comparative vigour and the attempted assassination attempt strengthened his support.
With increasing Democrat desperation, the pressure mounted and Mr Biden finally folded. It is a move he should have made weeks ago.
Suddenly, there is the need for a replacement, increasingly likely to be Mr Biden’s loyal vice-president Kamala Harris. She has had the endorsement of Mr Biden and the Clintons.
As of last night, all that crucially remained on that front was backing from former president Barak Obama and leading Democrat Nancy Pelosi.
While Ms Harris has polled poorly, her role as vice-president has been to be secondary to Mr Biden. She also earns kudos for remaining loyal to Mr Biden to the end.
Now, it has all changed and she has the chance to shine. Now, there is uncertainty in the race for the White House.
Now, there is a clear alternative to elderly white men. Now, the Democrats have the opportunity to muster enthusiasm and much more than just negativity about Mr Trump.
She is comfortable across different worlds and embodies characteristics that progressives should welcome. Imagine, too, the optics if she was overturned for a traditional male candidate.
The fact that she has been criticised for being insufficiently progressive on some issues should work in her favour. It is, always, the middle voters — and in the United States those in the swing states — who decide the result in the end.
It will be vital for activists and left-wing Democrats to listen outside their echo chambers and recognise advancement requires wide support and steady evolution.
Ms Harris has also been playing a leading role in the fight for women’s reproductive rights as various states knock them back in the wake of the Supreme Court’s overturning of Roe v Wade. Immigration, in contrast, will be a weak point.
President Biden was elected as safe hands and someone who would not scare off the semi-conservative. Yet, he still achieved progress on social, economic and climate fronts. He was able, as well, to return steadiness to US foreign policy.
He first became a senator in 1972, and his service to his country in various roles has been extraordinary.
Some options to Ms Harris see that a swift and united replacement of Mr Biden is in order and have or are declaring they will not oppose her. The Democrats have little more than 100 days until the election and must go for it with purpose and vitality. A messy selection process and convention would play into Republican hands.
There is much speculation on how successfully Ms Harris, a former prosecutor, will make the case against Mr Trump. Despite Mr Trump’s fervent following, the Democrats will seek to win support from potential anybody-but-Trump voters.
Assuming Ms Harris is nominated, and a television debate can be arranged, that confrontation between her and Mr Trump will be watched with phenomenal interest.
The narrative of Mr Trump as the younger man can now flip. His opponent is likely to be a full generation younger. More of the focus could be on his regular gaffes.
Mr Trump remains the frontrunner against the relatively unknown Ms Harris or another candidate. Nevertheless, the race is again all on and full of uncertainty.
