Johnson tried to make a comeback and flew home early from his holiday in the Caribbean upon hearing of the leadership vacancy, but he unexpectedly withdrew from the contest within just two days, on Sunday.
Then on Monday, Duncan Smith explained what he believed had pushed Johnson – who only left Downing Street himself on September 6 – to pull out of the race.
Speaking to LBC’s Andrew Marr, Duncan Smith claimed Johnson effectively tried to “bulldoze” Sunak into backing him during their crisis talks on Saturday evening – but that Sunak had pushed back.
Marr asked: ″Is that evidence of a much tougher minded man than we’d seen in public?”
“He’s certainly a tough-minded man,” Duncan Smith replied. “I know him, I’ve kind of half negotiated with him on bits and pieces, so I think he is tough – he’s always been like that.”
But, he also indicated Johnson’s response to the whole contest had been an obstacle to the former PM.
Duncan Smith said: “I think the problem when Boris came over was that Boris was completely, unexpectedly having to do this.
“He made no plans, he’d got no team.
“He kind of expected, I think, when he arrived there would be at least 150 people claiming him and that would grow to the majority.”
The threshold for any nominees to be contenders in the contest was 100 MPs. Johnson later claimed he had 102 MP nominations, but the BBC tally counted only 58 public backers.
Duncan Smith continued: “He certainly found himself struggling and begging for votes. That was demeaning, really.
″And then, when Rishi and the others said no, the only deal we would do with you is if you were serving us and not the other way around.
″That of course, didn’t suit him.”
Sunak served as Johnson’s chancellor for more than two years during the pandemic.
However, there is thought to be little love lost between the two after Sunak’s resignation in July triggered the downfall of Johnson’s time as prime minister.
Duncan Smith also had an optimistic prediction for Sunak’s early days in office, claiming he would enjoy a “period where people want him to succeed and get on with it”.
″As I say, everyone realised we cannot go on arguing.”
He also claimed that the 2019 mandate the Conservative Party won was not just one person’s.
This was a slight dig at Johnson’s previous claim he was “uniquely placed to avert a general election” and to lead the Tories, having secured them that landslide victory three years ago.
In a statement released on Sunday, Johnson said: “I believe I am well placed to deliver a Conservative victory in 2024 – and tonight I can confirm that I have cleared the very high hurdle of 102 nominations, including a proposer and a seconder, and I could put my nomination in tomorrow.
“There isa very good chance that I would be successful in the election with Conservative Party members.”
However, he added: “But in the course of the last days I have sadly come to the conclusion that this would simply not be the right thing to do.”
Johnson did eventually tweet his congratulations to Sunak once he was officially invited to create a government by the King. It came 22 hours after the announcement that Sunak was the new leader of the Tory party and de facto prime minister.