The demise of Boris Johnson had already put an extra spring in his step before the Government's mishandling of the economy sent him soaring up to cloud nine.
In recent days he has taken to referring to Chancellor Kwasi Kwarteng's mini-budget as "kamikaze" – a word he used to describe it on at least 50 occasions during media interviews on Thursday.
Indeed, it took just seconds for the term to leave his mouth during a roundtable with mortgage brokers at Acorns To Oaks Financial Services in Bilston, where Sir Keir was a guest of Wolverhampton South East MP Pat McFadden.
But despite the 'easy win' for Labour gifted by the mini-budget, Sir Keir said he had been left furious by its impact.
He accused Liz Truss of driving the economy "into a wall", and said thousands of people across this region will be left struggling to make mortgage repayments.
"I'm really angry," he told the Star in an interview.
"This isn't an Oxford debating society issue or a balloon debate. As a direct consequence of that mini-budget, people in this office have told me they have got to take life-changing decisions as to whether they can buy a house, whether they continue in the house they are in or even have a baby.
"It isn't a world event that has caused this. This is an irresponsible government that has had a direct impact on people in Wolverhampton and it is costing them hundreds of pounds a month.
"Am I angry about it? You bet I am."
Like the Prime Minister, Sir Keir believes that "growth, growth, growth" is the way to boost the UK economy, although he has a very different set of policies in mind to achieve that aim.
In fact, the very notion that the Government's plans would result in growth was "ridiculous", he said. He told the Star that Labour would start by reversing the mini-budget, while a windfall tax would be brought in to pay towards the freezing of energy bills.
Meanwhile, a new energy generation company, GB Energy, would ensure that the rewards can be reinvested in areas like the Black Country and Staffordshire, he added.
The economy was not the only tough issue Sir Keir was keen to show he could deal with in government.
He is already looking ahead to the next general election, where Labour will be aiming to regain seats in the Black Country that were lost in the Tory takeover of 2019.
With a massive lead in the polls he clearly smells blood, but Sir Keir is quick to say he is not taking anything for granted. He said he wanted people to "take another look at Labour", saying the party had "changed" over the last two-and-a-half years.
"We're now in a position where we are making the running," he said, suggesting the years of playing catch up with the Tories were over.
"What I say to people is look at what we're doing and listen to what we're saying. I know I have to earn every single vote across the West Midlands and that is what I will set out to do."
He conceded that his party would need to show it had strong policies across the board if it is to win back 'Red Wall' seats in areas such as the Black Country – including on immigration.
Asked about Liz Truss's comment in her conference speech that Labour had "no plan" to deal with illegal immigration, Sir Keir said: "The Prime Minister was talking nonsense – and not for the first time.
"In relation to the Channel crossings, nobody wants to see those journeys being made and what we need is a serious response to that.
"That starts with the upstream work that is needed to deal with the smuggling gangs. That requires a strong National Crime Agency (NCA) working with the French authorities and other authorities.
"If you can believe it, while the Government was criticising others, it was looking at cuts to the NCA, the only body that can only do this upstream work."
The Labour leader also insisted there would be "no chance" of Britain attempting to rejoin the EU under his watch.
He said: "We will make Brexit work, but that does not mean re-entering the EU, it means making the deal we have got work.
"All that we got out of this government was leaving the EU. We haven't got a deal that works properly for those individuals and businesses across the Black Country.
"I can categorically say we will not be going back into the EU."
With his party seemingly on the crest of a wave, Sir Keir cut a relaxed figure away from the cameras, talking about his love of Greggs vegan sausage rolls and of his joy at Arsenal's resurgence under Mikel Arteta.
"I really think they're heading in the right direction," he said.
Perhaps he was also talking about his party.