Here is one budget secret that George Osborne will not reveal when he stands up before the Commons at 12.30pm on Wednesday. During his day in the sun, when the chancellor must try to sustain the pretence that he has an unshakeable grip on the nation's economic destiny, he will not admit to us that few budgets matter a lot and many budgets don't matter much at all.

For every budget of consequence (the Howe deflationary budget of 1981 or the Lawson inflationary budget of 1988), there are several more when the man at the helm does little more than nudge at the tiller and tweak the sails.

His first – the self-styled "emergency budget" in June 2010, which committed the coalition to a long period of austerity – was one for the history books. Whatever the ultimate verdict on its economic judgment, that was a political success in persuading many observers to agree with the chancellor's definition of the central challenge. By making deficit-reduction the coalition's lodestar, he lashed the Lib Dems to his mast and placed Labour, which would prefer the emphasis to be on jobs and growth, in a corner from which the opposition has yet to escape. By contrast, the second Osborne budget and the Brown-flavoured mini-budgets that he has offered each autumn were largely forgotten within 24 hours of being delivered.

To which category will this Wednesday's performance belong?

If we judge it by the arguments that have been semi-publicly raging between the coalition parties, then this is going to be a highly significant budget. Yet, in some ways, the wrangling between the Tories and Lib Dems over taxes has been a distraction. The media naturally tend to focus on differences between the parties rather than agreements, but what the coalition has not fought about in advance of this budget is probably more important than the battles that have been waged. The big non-row has been about the deficit-reduction strategy. About that, there has been virtually no internal debate at all. They plan no deviation from the squeeze that is currently scheduled to last two years into the next parliament. The economy looks a bit stronger than it did at the end of last year. Business confidence appears to be ticking up a little. The interest on UK debt remains at rock bottom lows – so depressed that the Treasury thinks it might even tempt investors to buy 100-year bonds.

But unemployment has just hit a 17-year high and levels of joblessness among the young are especially dire. A further tightening, called for by those right-wing Tories who are unimpressed by the speed with which the government is cutting cannot be countenanced when a fragile economy remains so vulnerable to external shocks. A loosening cannot be risked, or so Mr Osborne will argue, because the bond market vigilantes are ready to sink their teeth into any government that shows infirmity of purpose in reducing deficits and international credit rating agencies are breathing down his neck. He has already answered the big macroeconomic question by saying that his deficit strategy is not going to alter. In fiscal terms, it will be a "neutral" budget.

By contrast, the political content of this budget will be far from "neutral". The chancellor, who doubles up as the prime minister's senior political strategist, already has at least one eye on the next Tory election campaign. So, too, do the Lib Dems, their unpopularity making them increasingly desperate to show that they are forcing concessions from the Tories. The lead-up to the budget has exposed clashing views between the parties, and within them, about who most deserves help.

The easiest bit of the bargaining was winning Tory agreement to Nick Clegg's demand that the coalition moves faster in raising the threshold at which income tax is paid to £10,000. The chancellor has always liked this policy and has even more need to be seen helping people on low incomes if he is to try to sell cuts in the top rate of tax as at all fair. He was initially less sympathetic to the demand that he should soften the impact of the withdrawal of child benefit from households in which at least one earner pays more than basic rate income tax. But since the strongest voice making that demand belonged to the prime minister, the chancellor has had to bend to it. On top of that, Mr Osborne has had his own priorities, among which a very high one was reducing the 50p top rate of tax.

Where to find the money to satisfy these rival ambitions has been at the heart of the hard bargaining over this budget. For every sweetener desired by the prime minister, chancellor or Lib Dem leader, a hit had to be taken elsewhere. The mathematics and politics of this budget have been further complicated by the warring instincts within David Cameron, which has led him to make somewhat contradictory demands on his next-door neighbour. The traditional Tory side of the prime minister is instinctively averse to asking for more from the wealthy. That is in conflict with the tactical centrist in David Cameron who is worried about doing anything that entrenches the reputation of the Conservative party as a party of the rich.

For weeks now, the budget has been battled over at Quad meetings of Messrs Cameron, Osborne, Clegg and the chancellor's Lib Dem deputy, Danny Alexander. A Quad conference telephone call on Friday afternoon agreed some significant elements of the budget, but it is wrong to suggest that this means everything is done and dusted. I am told that some of the arguments are "going down to the wire" and final decisions will not be made until Monday. That is as late as they can leave it because the numbers have to go to the invigilators at the Office of Budget Responsibility.

The Lib Dems are not going to get their mansion tax and probably knew from the start that the Tories were unlikely to be persuadable. George Osborne could see the intellectual case for taxing wealth via property and some Treasury officials were attracted to the simplicity of a tax that would be hard to avoid. The chancellor might have been willing to cut a deal with the Lib Dems, but the prime minister was not. David Cameron feared the reaction of Tory MPs and the Tory core vote, among whom are rather a lot of people living in the size of property that would attract the tax. I am told that during one discussion of the mansion tax, Mr Cameron exclaimed: "Imagine what Labour could do with it." He also had a rather cruder, short-term electoral consideration that has been surprisingly overlooked in all the debate about the pros and cons of a mansion tax. Many of the homes worth £2 million or more are concentrated in London. There is an election for mayor of the capital coming up very soon. David Cameron did not want to do anything that could be said to jeopardise Boris Johnson's chances of beating Ken Livingstone. The first thing that a defeated Boris would do would be to try to get back into the Commons, which is the last place that Mr Cameron wants to see his fellow old Etonian. A beaten Boris will be bad enough for the Tories; a martyred Boris able to blame his defeat on the prime minister and the chancellor would be much worse for them. So the mansion tax was blocked.

Usually reliable sources tell me confidently that Mr Clegg has made much more progress in convincing the Conservatives of the merits of what the Lib Dem leader likes to call a "tycoon tax". The Lib Dem leader floated this idea in an interview last weekend and it is probably safe to assume that he would not have gone public with it 10 days before the budget if he did not think he was going to get some sort of win.

As things look this weekend, the Tories have agreed to put a legislative cap on how much the wealthy can use allowances and avoidance to escape tax. In exchange, the Lib Dems have signed off on a cut in the top rate to 45p.

That represents a very big political gamble on the part of both coalition partners. Reducing the top rate will please a lot of Tories and it won't have escaped Mr Osborne's notice that these are the people who will ultimately select Mr Cameron's successor. It will obviously go down well among the minority who earn enough to pay the top rate. No doubt it will be justified on the grounds that a 50p rate sends a negative signal to entrepreneurs and deters talented people from working in the UK. But the chancellor will struggle to explain why he has made a priority of cutting the top rate to the far greater number of less affluent voters who are suffering the worst squeeze on their living standards in decades.

Senior Labour figures, one of whom describes it as "politically mad", are torn between not being able to quite believe that he will really do it and desperately hoping that he does. If this is not to be seen as a budget by the rich for the rich, the chancellor will have to be utterly convincing that he will be asking for more from the affluent in other ways. If he is not, the country will know that when he next says: "We're all in this together", he means everyone except the wealthy.