And as he bounced from scandal to scandal in the primary, the prevailing wisdom was that Republicans would coalesce around another candidate.
But Trump defied expectations and shocked even himself by winning the White House in 2016.
So can he do it again in 2024?
The first step is winning the Republican primary, which is not a sure thing.
The case for Trump winning the nomination
Running for president is a humbling experience for politicians not used to being humbled.
The primary process involves going to small towns in Iowa and New Hampshire and convincing voters, often one at a time, that they should be president.
This is not going to be a problem for Donald Trump.
Trump already has universal name recognition and a devoted fan base. They may not account for more than half of Republican primary voters, but they will show up in enthusiastic force to his rallies.
While his rivals try to shake hands with shoppers outside suburban strip malls, Trump remains a big deal.
And while being below 50 per cent could be seen as a sign of weakness, Trump only needs to win over a few more people to win big.
His fan base also gives him a powerful grassroots fundraising base. While his rivals may struggle, Trump could blanket the airwaves with ads.
But Trump’s greatest advantage is the simple fact that he’s done it before, and he knows how to do it.
And Governor Scott Walker, who had become a conservative hero in Wisconsin, struggled to respond when Trump attacked his economic record, and his campaign never got off the ground.
The case against Trump winning the primary
In 2020, Republicans did not dare criticise their president, for fear of aggravating his army of devoted supporters.
But two years on, it appears there are a few politicians who aren’t afraid of challenging Trump – and some of them are top members of his administration.
That includes former Vice President Mike Pence, who is considering his own bid for the White House.
Pence has published a memoir and is doing a media tour, the archetypal schedule of someone about to run for president.
Pence has been loath to speak out against Trump in the past, but his new book is not a flattering view of his former boss.
Meanwhile, Trump’s ambassador to the UN, Nikki Haley, has also hinted at her own run.
“A lot of people have asked if I’m going to run for president now that the midterms are over,” she said.
“I’ll look at it in a serious way and I’ll have more to say soon.”
And former Secretary of State Mike Pompeo has confirmed he is considering a tilt.
“We need more seriousness, we need less noise, we need steady hands, we need leaders that are looking forward, not staring in the rearview mirror claiming victimhood,” he said.
DeSantis is in many ways the perfect Republican foil – young, popular in a crucial state, and in many ways more conservative than Trump.
If the Republican never-Trumpers coalesce around one candidate, it will probably be DeSantis.
But DeSantis stands a better chance of the presidency if he waits until 2028.
Because in the words of The Wire‘s fearsome character Omar, “if you come at the king, you best not miss”.
Donald Trump announces he’s running for president in 2024