Why has Labour U-turned on its green investment pledge?

The promise was clear. And it was prominent.

At Labour's 2021 conference, shadow chancellor Rachel Reeves announced her ambition to be the UK's first "green" chancellor.

To stress her bona fides, she pledged to invest £28bn a year, every year to 2030 to "green" the economy.

Labour's Green Prosperity Plan was one of its defining policies. It gave the party a clear dividing line with government.

Ms Reeves said there would be "no dither, and no delay" in tackling the climate crisis.

It was also an answer to the government's "levelling up" pledge.

The borrowed cash would underpin well-paid jobs in every corner of the UK in the energy sector.

So why has Ms Reeves kicked the pledge into the second half of the next Parliament, if Labour wins?

Why has a £28bn pledge effectively been reduced to a target?

The first reason is obvious.

Ms Reeves now says she was "green" - in a different sense of the word - in 2021, in that she hadn't foreseen what then-Prime Minister Liz Truss would do to the economy.

And indeed the political and economic landscape has become bumpier with a war in Europe and a cost-of-living crisis.

With interest rates up, the cost of borrowing rises too, making the £28bn pledge more expensive to deliver.

And Ms Reeves wants to emphasise that if any spending commitments clash with her fiscal rules, the rules would win every time.

But did the £28bn green pledge really clash with her rules?

In their own detailed briefing on their fiscal rules, Labour said: "It is essential that for our future prosperity that we retain the ability to borrow for investing in capital projects which over time will pay for themselves.

"Labour recognises the need for investment which raises the growth rate of our economy by increasing productivity as well as stimulating demand.

"And that is why our target for eliminating the deficit excludes investment."

So borrowing to invest in the future technology and jobs shouldn't fall foul of that fiscal rule.

But there is another rule which Ms Reeves cited this morning - to have debt falling as a percentage of GDP or Gross Domestic Product, a measure of economic activity.

Meeting that rule may have contributed to putting the £28bn on the backburner - though I remember at the 2021 conference some senior Labour figures questioning the wisdom of borrowing the equivalent of half the defence budget every year even then.

And some senior figures in Labour are far less convinced that £28bn would necessarily bust the debt rule - economic forecasts can change by far greater margins.

One of the other justifications for the change of position is that £28bn shouldn't be poured in to the economy straight away.

That's because it will take time to train workers, to create and bolster supply chains. Hence "ramping up" to £28bn.

One shadow minister said that while today's announcement felt like a bit of a handbrake turn, it was nonetheless inevitable and sensible.

The scale of the ambition remained the same, but pragmatically the shadow chancellor was simply not committing to spending which would be difficult to deliver.

But all this must have been known in 2021, too.

So why announce the U-turn today?

The change of position was discussed within Labour's Treasury team for some time.

Engagement with investors convinced them that there was enthusiasm for green industries so the government itself may not need to pump in a huge amount of cash straight away - the private sector would provide some of the new green jobs without state help.

And while Ms Reeves has ditched the £28bn pledge in the first half of the Parliament, this doesn't mean that a Labour government would spend nothing on its Green Prosperity Plan.

I understand cash will be prioritised for projects where the private sector would not commit without state assistance - nuclear and hydrogen for example.

The volatility in the markets has also concentrated minds when it comes to making firm promises based on borrowing.

But it seems clear that politics and not just economics played a role in today's announcement.

There have been grumbles and growls over how the policy has landed over the past two years within Labour's ranks and internal criticism has increased, not receded.

One concern was that the amount to be borrowed - the £28bn - was better known than what the money would buy - from home insulation and heat pumps to new carbon capture technology.

But it was crystal clear this week that the Conservatives felt that they had seen a vulnerability that could be exploited.

The front page of the Daily Mail blared this week about the alleged dangers of the policy - the extra borrowing would put up interest and therefore mortgage costs.

The independent Institute for Fiscal Studies was also being cited by Conservative ministers.

Its director Paul Johnson had warned that while additional borrowing would pump money in to the economy, it also drives up interest rates.

As Labour has been attacking the Conservatives for their handling of the economy, and the "mortgage premium" they claim the government has caused, it was understandable that they did not want the same attack to be aimed at them, and Ms Reeves this morning sought to eliminate a potential negative.

As one Labour shadow minister put it: "They [the Conservatives] will be pulling their hair out that one of their attack lines has failed."

Some in Labour's ranks, though, believe the party should have insulated (no pun intended) itself from attack by making the case more stridently that borrowing to invest is different from borrowing to meet day-to-day spending.

Labour's opinion poll lead is wide but pessimists in their ranks fear it is shallow.

Establishing economic credibility is seen as key.

But while it may have been the lesser of two evils, today's change of tack isn't cost-free.

The party has committed to achieve a net-zero power system by 2030.

But with potentially significantly less investment, is this target in danger too?

And unlike many of the left-wing commitments that have been ditched - where the leadership don't really mind the backlash - this was the shadow chancellor watering down her own highest-profile pledge.

That in itself has allowed the Conservatives to shout about Labour's economic plans being "in tatters".

As Labour is still committed to its Green Prosperity Plan - just not the original timescale - they will still claim they have clear dividing lines with the government.

But one of their key arguments has been this: With the US pouring subsidies in to domestic green industries, the UK will get left behind if it doesn't follow suit. And fast.

A delay doesn't destroy - but it does potentially weaken - the Labour case.

But there is another concern amongst those who are most certainly not on the Corbyn left.

Emphasising competence and fiscal credibility over climate change commitments could leave some target voters cold.