The man who hoped to overturn Brexit in a second referendum is now categorically and clearly at peace with it.
This is a huge turnaround from Sir Keir, who three years ago attended a rally to support what was called a People's Vote. He hoped for a victory for Remain.
He says that he would not advocate joining the European Union as prime minister, or signing up to three of its most important ideas: the single-market, the customs union, and freedom of movement.
This means that Sir Keir does not accept Brexit. Instead, he is advocating for a "hard Brexit", which many critics like to call it. He argued that the government's vision was better than the EU's.
So why is he saying that?
I felt like he was following Lynton Crosby's advice to David Cameron, the former Prime Minister, to "get your barnacles out of the boat".
Also, get rid of the things that are preventing you from focusing on the important stuff.
Sir Keir's focus in the coming months and weeks is on what he believes is at the root cause of many of the country’s problems: a lack economic growth.
He stated that "our driving mission is to get Britain’s economy growing again."
Although the Labour leader has presented his new view on Brexit in a more fragmented fashion before, the speech at the Irish Embassy and interviews with the political editors of Sky, ITV, and Sky still felt like a moment.
After much long-term anguish and disagreement over Brexit, this was the leader and sole representative of the main opposition party at Westminster. He accepted it and sought to move on.
In a series of consecutive sentences, I have nailed that the arguments about Brexit that have so dominated British political discourse for years are, at the very least, over.
It does raise the question of authenticity. What does he really believe? Is it what he said three-years ago or what he says now?
He said, "We have quit the EU now, there is no going back," to answer my question. He argued that accepting these fundamentals would allow him to focus on making Brexit work better, especially in Northern Ireland.
It is not surprising that all of this has caused some internal squirming.
This is an organization where most of its members are passionate supporters of Remain and deeply skeptical about Brexit.
Sadiq Khan, the Mayor of London, stated that it would be worthwhile to join the EU's single Market, which aims at making trade between countries as simple as within them.
You might also imagine that, given the Brexit referendum's split of 52% to 48% for Leave, there may be lots of opportunity for Labour to win out by appealing to Remain voters.
However, Remain votes were concentrated in certain areas, while Leave votes were more evenly spread across the country.
So, a large majority voted Leave.
There are 632 constituencies across England, Scotland, and Wales.
403. Of these, 229 voted for Remain, while 229 voted Leave.
The Conservatives won 72 Remain seats, while Labour is the closest challenger in only 30.
Labour cannot appeal to only Remain-voting regions, so it can't make any progress.
If there was a significant shift in opinion towards Brexit, things might have been different.
However, polling data suggests that more people believe voting to leave EU is a bad decision. There is still a substantial split, so it is likely that in many constituencies, staying out of EU is the preferred option.
It's different for the Scottish National Party, Green Party, and, at least in part, the Liberal Democrats.
The best chance for the Greens and the SNP to win the next general election is in Remain-voting regions. They will try to capitalize on this.
Already, the Green Party claimed that Sir Keir's position is driven by Labour's selfish interest rather than the best interests the country.
The Lib Dems will be looking for Conservative-leaning seats that are Conservative-held and Remain-leaning. However, recent by-elections held in Devon, Shropshire, and elsewhere have demonstrated they can win in Leave leaning areas.
However, Labour's biggest challenge is to win seats currently held by Conservatives, especially those it lost in 2019 for the first-time in decades where there was a strong Leave vote in 2016.
The party hopes that by accepting Brexit unambiguously, it will be able to get a second hearing for Sir Keir's next economic vision.
He needs to just pray that he gets the chance. Because he has already promised to resign if there is a Covid violation fine, but he still awaits the Durham police decision.