Treasury wants to cut pensions tax break
Plainly the big story of the Budget will be the deterioration of Britain's public finances, with the government's borrowing needs exploding to record levels for peacetime.
But tacked on to that hair-raising tale will be a subsidiary story, which will be the public-spending cuts and tax rises required to bring the government's expenditure and revenue nearer to equality over the coming years (though we're only going to get chapter one of this epic).
That's why in the pensions industry, which benefits from tax breaks and is habitually a target when money is tight, there is a great deal of nervousness.
For years, many in the Treasury have taken the view that providing full tax relief on pension contributions to top-rate taxpayers is not a sensible use of scarce resources - and in the past few weeks, ministers and officials have been sending out signals that the moment may be nigh for reform.
So some kind of change to the tax breaks available to high earners on their pension contributions is under active consideration.
That said, the chancellor is unlikely to decide till the eve of Wednesday's Budget whether to abolish the right of all top-rate taxpayers to offset their 40% tax liability against what they put into their pension pots.
The reason for prevarication is that limiting tax relief to the 20% basic rate of tax would represent a substantial rise in the tax burden on many hundreds of thousands of people who don't think of themselves as rich.
It would bite, for example, on those earning just a bit more than £40,000 a year, including deputy head teachers and senior policemen. The cost for many of them of limiting the relief to 20% could be more than £500 a year, equivalent to a 1% rise in their tax rate.
Which is why the Tories would be certain to oppose it - and it's already clear from newspaper coverage that the Tories would have a following wind from commentary and coverage in much of the press.
So what would be the argument for abolishing the relief, other than that the government needs more revenue (abolition would raise more than £5bn a year, a non-trivial sum)?
Well the Treasury would say it's slightly odd that the biggest tax breaks on pension contributions go to that part of the population who are saving adequately for retirement, whereas the smallest tax incentives go to the vast majority who are not saving enough.
As a nation we're not putting aside sufficient amounts for retirement - and the greatest deficiency in saving is at the bottom end of the income scale (as you'd expect).
So some would say it's a bit rum that for every £5,000 put into a pension pot by a top-rate taxpayer there's a refund of £2,000, whereas the refund on the same contribution would be just £1,000 for a basic-rate taxpayer.
Is that fair - especially when there are more than 23m basic-rate taxpayers and less than 4m paying the top rate of 40%?
You'd think, from much media coverage, that the vast majority pay 40% tax, and would therefore be hurt by abolition of 40% relief. But that's not so.
That said, would reducing financial support for those who are making decent contributions do anything positive to fill the horrible hole in final-salary pension schemes or boost the meagre payments into defined-contribution plans?
If one of the biggest structural problems faced by the British economy (and those of most other rich countries) is the under-funded pension burden of millions who are living longer, does it make economic sense to withdraw one form of support from pension contributions, without providing other incentives to save?
The chancellor could go for a more modest reform, limiting full 40% relief to earnings up to £100,000 or so.
Which wouldn't raise as much for the Treasury, but there probably wouldn't be opposition from David Cameron's Tory party, because he has made a strategic decision that he can't be seen to be defending the interests of those on highest incomes.
The argument here will also go wider than just the future of our pensions.
There's also a debate about the relative sizes of the public and private sectors.
It's moot whether right now the consensus would be that the hole in the public finances should be filled predominantly by shrinking the state or by finding ways to increase tax revenues.