Responding to the Budget is one of the toughest tasks facing any Leader of the Opposition.
The Chancellor sets out their plans for the economy and the public finances, having weighed up the pros and cons of every measure following weeks of detailed discussions with political colleagues, officials and hearing representations from a range of interested parties.
In contrast, the Opposition leader has just the hour or so their opposite number is speaking to consume a wealth of complex data, consider both the political and economic implications and construct a coherent response.
All they can rely on is their own wit and political antennae – as well as the notes frantically scribbled by helpful (although not in all cases) colleagues – to survive this high-wire act rivalled only in the political world by TV election debates.
But the job is even harder when they are merely keeping the chair warm for someone else.
Ms Harman, a veteran of frontline politics, undoubtedly had strident thoughts of her own to offer on the first truly Conservative Budget for 18 years. However, as the mere stand-in, she had no authority to utter them and risk binding her party to positions that her successor could reverse.
Instead, she found herself trapped in a political vacuum between Labour’s failed election platform and the unknown direction of the party under the next leader which, given the candidates, could range from a distinctly more Blairite agenda to something more closely aligned to Greece’s Syriza party.
This helps explain why this normally-confident performer found herself telling MPs that Labour would oppose the Government’s bad ideas and support the good ones.
Well, one would hope so.
She condemned George Osborne for setting out a plan to cut welfare without offering proposals on raising wages – having just listened to the Chancellor announce a new national living wage. It was clearly a pre-prepared line, rendered useless when Mr Osborne used the closing lines of his address to announce his flagship policy.
Similarly, her calls for consultations on plans to change Sunday trading laws – leaked by the Treasury earlier in the week – made her look as if she was responding to old news.
Not only did Ms Harman’s own curious position conspire against her, but she was also facing a politician who doesn’t decide which side of the bed to get out of until he has weighed up the political implications.
This was a Budget littered with dividing lines on key issues such as tax, welfare and wages designed to force Labour into ideologically difficult positions.
Not content with putting the boot into Labour as it recovers from election defeat, Mr Osborne has already laid a trap for its new leader who will have to decide within weeks of assuming the post whether to back Mr Osborne’s Fiscal Charter – committing future governments to a budget surplus in “normal times” – which will be put to a Commons vote this autumn.
But in one, very important respect, Ms Harman landed a blow on the Chancellor.
For years, George Osborne trumpeted his record on infrastructure spending, with transport schemes in particular announced and re-announced to underline his commitment to a so-called Northern Powerhouse.
However this Budget was noticeable for its dearth of such trinkets aimed squarely at persuading journalists like me to write that the Chancellor is “delivering for Yorkshire”.
There were just two instances when Mr Osborne uttered the words “Northern Powerhouse”. The reason? Pure and simply, he did not want to draw fresh attention to an embarassing failure.
Having spent the election campaign talking up investment in the North, the Government shelved proposals two weeks ago to electrify key rail routes connecting Yorkshire to the North West and London.
Ms Harman was right when she warned the “great Northern Powerhouse is starting to look like the great Northern Power Cut”. When it comes to delivering this vision, Mr Osborne is in danger of becoming the “jam tomorrow” Chancellor.
Putting rail electrification on hold has robbed Yorkshire of one of the few tangible examples of the Northern Powerhouse plan in action. In their absence, what is left is a lot of debate about long-term projects – such as HS2 and a new trans-Pennine road link – that will take years to deliver.
The Chancellor is being presented with ambitious plans for devolution to Yorkshire and if he wants to demonstrate a genuine commitment to the wider One Nation agenda, he should respond positively.
In correctly striving to tip the balance from welfare to work, he must not forget his commitment to rebalance the economy from South to North.
James Reed is political correspondent of The Yorkshire Post.