The only figure that matters for Osborne this week is the majority of 12

George Osborne prepares for what is in effect his fourth Budget in 12 months. There was his pre-election Budget, his summer Budget, his Autumn Statement – a budget under a different title – and now his spring Budget.

Be prepared once again for another torrent of figures, growth projections, deficit forecasts and more proposed spending cuts. All these figures will not connect very much with those spilling forth at previous Budgets. Dependent on global trends, the Chancellor’s game-playing, his ideological inclination and mood of Conservative MPs at any given time – they will all be subject to immediate revision.

There is only one reliable figure to contemplate this week, one that will determine economic policy for the next four years. That figure is 12 – the size of the Government’s overall majority, and a figure that is likely to fall, if it changes at all, between now and 2020.

The unexpected result at the 2015 general election obscured the fact that the Conservatives secured a tiny, constraining majority. That majority becomes more fragile when the unity of the Conservative parliamentary party is under strain. To note that during the referendum campaign (and perhaps for long afterwards) party unity is under strain is one of the more understated ways of describing the mood among Conservative MPs.

The combination of a tiny majority and the urgent need not to alienate voters before the referendum means that George Osborne is even more trapped than a Chancellor normally is, especially at this stage of the parliament. The spring Budget after an election victory is when a Chancellor can make big policy changes that risk being unpopular in the short term but are subsequently recognised as epoch-changing announcements.

Entire books have been written about Geoffrey Howe’s 1980 Budget. In 1988 Nigel Lawson slashed income tax for high earners among, several sweeping reforms. In 2002 Gordon Brown openly increased taxes to pay for NHS spending. All were controversial Budgets of historic significance. For the first time since becoming Chancellor, Osborne contemplates a post-election spring Budget with an overall majority. Yet with the referendum looming, he has to approach his address as if it were a pre-election Budget. Precisely at the point when chancellors are normally free, Osborne is incarcerated.

Wisely, his aides briefed the press well in advance that he had dropped all plans for a substantial reform of pensions. This might have been a genuinely progressive change, and Osborne likes to deploy the term “progressive” as often as possible. Indeed, he applied the adjective when cutting tax credits on the low paid during the summer and it would be useful, for him and us, if policy and adjective matched every now and again. But there is no point blaming Osborne for the decision not to press ahead with changes to pension tax relief. In not acting he took the only route available to him: this is not the moment to alienate the affluent elderly, for they are the types who tend to vote and the make-or-break EU referendum looms. Even if he had wanted to take the risk, there were already enough Tory MPs ready to oppose the reforms. It only takes six or seven and Osborne risks defeat. Lawson and Brown made their ambitious moves with landslide majorities to support them.

Ominously, Osborne has indicated that he will propose further spending cuts with so many revenue raising options closed off. His former head of staff, Rupert Harrison, notes in the Financial Times: “There is little political cost from pencilling in further spending reductions, but there is much to be risked from tight parliamentary votes on higher taxes.”

The observation is carefully worded. Pencilling in further spending reductions does not necessarily mean they will all be enforced. Vague, unspecified cuts to be implemented at a future date are obviously less politically risky than tax rises now. Most voters approve of spending cuts in general. It is only when the reductions are applied to specific services that the problems can become urgent politically, economically and in human terms.

But there may have been more than six Conservative MPs who would have opposed possible changes to pensions. There will soon be many more than that complaining about the impact of specific spending cuts. From the tax credit reductions Osborne was forced to reverse, to the crises facing many hospitals and on to the need for ambitious house-building programmes, being miserly is not wise or politically safe.

Not that the cuts announced in the Budget will necessarily come to pass. If there is a whiff of political trouble, Osborne will change course or change the figures. In his pre-election Autumn Statement, Osborne produced spending projections that even the linguistically restrained Office for Budget Responsibility compared to “a rollercoaster”. By the time of his pre-election Budget, just a few months’ later, those figures had been conveniently revised. And in his first post-election Budget last summer, the growth and spending figures had been revised again in a spirit of buoyant optimism, but even so there was an apparently urgent need for cuts to the working poor tax credits; the revised figures in his most recent Autumn Statement meant that there was suddenly no longer any need for these previously urgent cuts.

The Chancellor has a referendum to win, a leadership contest to fight, and is partly powerless as global storms brew. This weeks’ torrent of figures will be revised before they have blown away. The majority of 12 is the only figure that will not change very much and means the Chancellor will be trapped in future Budgets, too.