For weeks, a narrative had held in Washington that Donald Trump was on an unstoppable romp back to the White House, with the November 5 election due to be little more than a formality to determine the extent of President Joe Biden's humiliation.
The sense of inevitability built as Mr Trump bounced from one victory to another, starting with the conservative Supreme Court ruling that all but ensured he won't face prosecution or punishment in the criminal cases against him.
Then, a debate packed with falsehoods, vitriol and wild claims was completely overshadowed by Mr Biden's incoherence.
And on July 13, Mr Trump narrowly escaped a would-be assassin's bullet, elevating him to a near-mythical status among Republicans, many of whom now see him as being anointed by God.
Contrasted against this seemingly inexorable trajectory, Mr Biden appeared determined to stay in the race, making a slow, zombie shuffle towards an electoral abyss that would forever have tarnished his legacy.
But the storyline changed completely on Sunday when Mr Biden made the heart-rending decision to step aside, ending his storied political career. He immediately endorsed Vice President Kamala Harris to be the Democratic nominee.
Suddenly, the game had changed again.
After spending months ambling along like prey animals resigned to being eaten, the Democrats have been given a jolt and an opportunity to regain ground in what has already been a bizarre and confounding election campaign.
The favourite to take the party's nomination is Ms Harris, who quickly won endorsements from Bill and Hillary Clinton.
Many Democrats are leery about Ms Harris, given her chequered performance as Vice President in which she has failed to have any breakthrough moments with the American public, while also drawing criticism for giving odd and rambling public statements.
I predict that the Democrats will quickly coalesce around Ms Harris, however.
Having a youngish (she's 59) black woman run against Mr Trump, who is 78 and white, might be the sort of catalyst disenchanted Democrats need to get out and vote on November 5.
Her relative youth flips the narrative on Mr Trump, who repeatedly attacked “Sleepy Joe” for his energy levels.
It is now his age that becomes the issue: he is the oldest candidate ever to seek the White House (albeit for a second time).
Without the shield provided by Mr Biden's decline, Mr Trump is now vulnerable to real scrutiny about his own physical and mental state. The former president frequently veers into random and odd digressions, including a bizarre rant about sharks and the risk of electrocution from boat batteries during a rally in Nevada.
Ms Harris, who is the first woman and first black and South Asian person to serve as Vice President, also provides a crucial contrast to Mr Trump and his white running mate JD Vance, both of whom want to deprive women of the right to choose whether they can terminate a pregnancy.
Democrats need to figure out if they are going to hold an open competition for the nomination at next month's convention, or if Ms Harris will be made a shoo-in given her credentials as Vice President.
An open convention would serve the Democrats well, as it would bring transparency to a party often perceived as being run by out-of-touch power brokers who rig the system in favour of establishment candidates such as Hillary Clinton in 2016 and Mr Biden in 2020.
Assuming Ms Harris does eventually become the Democratic nominee, who she picks as her own running mate will probably determine the fate of the race.
Several senior Democrat names have been floated, including California Governor Gavin Newsom, Kentucky Governor Andy Beshear and North Carolina Governor Roy Cooper.
Arizona Senator Mark Kelly, 60, is also in the mix. In my view, he would act as an interesting foil to the Republican ticket.
Where Mr Vance has been feted for serving as a military reporter in Iraq, Mr Kelly is a former naval aviator who flew combat missions in the Gulf War before becoming an astronaut.
And while Mr Trump was shot in the ear, Mr Kelly's wife, then-congresswoman Gabby Giffords, was shot in the head in an assassination attempt in Tucson, Arizona in 2011, and the couple have gone on to campaign for gun reform.
Whether Ms Harris and Mr Kelly will join forces is an open question, but Mr Biden's decision on Sunday gives the Democrats an opportunity to do a hard reset and challenge many of the assumptions of the Trump campaign.
They need to act quickly, as the Democratic National Convention starts on August 19.
