And so he should be. He is one of the most powerful post-war prime ministers this country has seen.

Having won a huge majority, he has now set out a huge programme for government in a King's Speech that contained 40 bills - double the average of such speeches since 2010.

Politics live: Starmer pledges 'national renewal' in King's Speech

Sir Keir knows the opportunity he has - to reshape Britain in a way not seen since the days of Margaret Thatcher or Tony Blair.

The other post-war leader who won such an opportunity at the ballot box was Boris Johnson, but he squandered his chance. This prime minister is intent on not wasting his.

To that end, this was a King's Speech that put growth at its heart, with 15 bills around this agenda - be it on safeguarding economic stability, increasing economic powers for metro mayors, driving through housebuilding, improving the rail and bus networks so people can get to work, or setting up Great British Energy to drive through more green power.

"We fought the election on change and to put forward a programme like this," explains one of Sir Keir's key allies.

"We have done loads of work preparing for this moment and we are determined to make the most of it. The three priorities in all of it is growth and economic stability, transport and then devolution."

It is also a King's Speech which gives us a better understanding of what "Starmerism" is.

Read more on King's Speech:
King's Speech key announcements
Votes at 16 absent from King's Speech
Sunak defends record in King's Speech exchanges with Starmer

As a set of values, it is about public service, the rule of law - he was the country's chief prosecutor after all - and, primarily, pragmatism. He came to politics late in life and seems light on ideological baggage.

As a set of policies, it is interventionist.

"You are seeing a return to believing that government has a role, not in trying to run everything but being a strategic centre," explains one architect of the government programme.

"It's not old fashioned in the sense of nationalising everything or leaving it up to the market.

"We are saying the role of the government is to set the strategic direction, provide stability and set the direction for the private sector to come in and then deliver, be it housing or green energy."

For the scale of the opportunity is matched only by the scale of the task. Announcements are the easy bit, delivery is much harder.

History is littered with past prime ministers who promised to grow the economy, "level up" the country, cut immigration or build more houses, only to fall short.

Prime ministers also know how easy it is to get blown off course.

His team tell me the big risks are the "unknowns". Who could have predicted the coronavirus pandemic or Ukraine war and the cost of living crisis that flowed from that?

There is also the question of what the inheritance is, with the prison crisis being one such problem that was bigger than Sir Keir had expected.

But perhaps the biggest risk is being in a situation where public expectations get ahead of where the prime minister is on delivery.

This is why Sir Keir is at pains to be truthful and not overclaim.

You will hear him, time and again, talk about changing the country being "serious work" that "requires patience".

There has been little flesh on the bones on his "first steps" of government precisely because he doesn't want to box himself in to timeframes on delivery that he fails to keep.

Instead, voters can expect progress reports on how the government is doing - be it on teacher recruitment, cutting NHS waiting lists, or improving rail and bus services.

? Click to subscribe to the Sky News Daily wherever you get your podcasts ?

There is also pushback - from within and without.

Already on Wednesday, some Labour MPs are publicly pushing Sir Keir to abolish the two-child cap on benefits.

The Labour leader is trying to hold the line, but with MPs prepared to table amendments to his King's Speech, he did use his address in the House of Commons to announce his child poverty reduction strategy.

John McDonnell, the former shadow chancellor, told me afterwards that the government had been "forced into bringing this announcement forward, and so prominently, because of the strength of feeling across the Parliamentary Labour Party".

But he added that it was "just a repeat of what has already been said and, as such, the pressure for scrapping the two-child limit will continue".

Meanwhile, on planning, there is already controversy over Labour's policy to force local councils to adopt mandatory housing targets within months.

Former housing secretary Michael Gove relaxed the targets last year amid pressure from Conservative backbenchers, but Sir Keir is determined to plough ahead in the face of probable local opposition.

All of it is difficult. But Sir Keir knows all about that. When he took over as Labour leader in 2020, most people thought he'd be a Neil Kinnock character, knocking the party back into shape but falling short of leading the government.

They underestimated him. He has to hope they will do so again.