These are taxing times for politicians. I don�t mean the daunting challenge of setting the Glasgow East by-election alight in the middle of the Glasgow Fair.

These are taxing times for politicians. I don't mean the daunting challenge of setting the Glasgow East by-election alight in the middle of the Glasgow Fair. Or the prospect of MPs having to trudge round Ikea to furnish that London flat, on a much-reduced and capped segment of their expenses. These are, quite literally, taxing times because setting new or higher taxes and making them stick is becoming harder and harder. And there is likely to be even stiffer public resistance to future moves in that direction as the credit crunch and the economic slowdown bite.

The Brown government has yet again postponed its planned 2p a litre increase in duty on petrol and diesel. Coming on top of the partial U-turn on the 10p starting rate of income tax and concessions to entrepreneurs on capital gains tax, it has heightened speculation that further sweeteners - on the new vehicle excise duty regime due next year or on stamp duty for cheaper homes, for instance - may soon follow. Once politicians blink on one unpopular tax rise, those affected get the message that, given enough pressure, they may well blink again. And again.

That's been the story on the fuel-duty escalator over the past decade. It was introduced, remember, by Ken Clarke at the fag-end of the Major government, when the global oil price was plunging towards $10 a barrel. Raising the duty on car fuel by more than inflation year-on-year had genuine green appeal back then, when the cost of oil was at rock bottom. But the idea hit the buffers after Labour came in and scrapped it as the price started to surge again and hauliers gathered, in September 2000, to blockade refineries in protest. The shock of how quickly Britain could grind to a halt has persisted. The 2p rise, now scrapped, would only have been the third since 2003. The Treasury says fuel duty is now 17% down in real terms, compared with where it stood in 1999.

Of course motorists don't stand there, filling up their tank, doing the mental arithmetic on how much of the price pain they are feeling is down to the soaring market price of a barrel of oil or the level of government taxes. The new Tory solution is what David Cameron calls a fair fuel stabiliser. As part of his economic recovery plan, the party which gave us the fuel-duty escalator would, if elected to power, introduce a system whereby, when the global oil price rises, around half any increase would be offset by a cut in duty. And when the oil price falls, the tax would go up again.

It has, like much of the Cameron canon, plenty of surface appeal. But it begs loads of tricky operational questions. Governments set tax rates annually.

As we've seen in the recent oil-price surge, the cost of a barrel can rise or fall by as much as $10 in a single day. Is the Tory leader proposing a fair fuel stabiliser which changes the rate of duty by the day, the week, or the month? Or would it only operate once a year? If duty rates tracked the oil price, what confidence can he or his chancellor have about how much revenue that particular tax is going to bring in over a year? If they've no idea, what does that mean for setting credible annual public spending plans? And if the stabiliser only kicks in once a year, at Budget time, just exactly how would that mitigate the rocketing cost of living in the meantime?

The Financial Times called such Cameron tax wheezes "feeble and expensive" this week. But to be fair to the Tory leader, he did admit taxes overall may have to go up if the Brown government loses power in 2010 and he is left with public finances stretched to breaking point. The omens are not good. The slump in house sales is bound to cut revenues from stamp duty. Some say they could be squeezed by as much as £5bn this year. The economic slowdown is bound to squeeze corporate profits. That means lower revenues from corporation tax. Any serious slowdown in the high street will hit VAT. And if more people find themselves out of work, the income tax take will also come under pressure.

The Tory leader reaffirmed this week that he will stick, if elected to power, with current Labour spending plans. They are already "tight", he tells us. And with higher inflation and unemployment creeping upwards, they "will become tighter still". But that begs another big question. The Cameron prescription is to share the proceeds of growth between higher spending and tax cuts. But if growth proves anaemic over the next couple of years, while public finances are so stretched that tax rises become inevitable, the concept of sharing the proceeds of growth is rendered virtually meaningless. Spending, even at Labour's tight legacy levels, surely has to contract.

That's what Nick Clegg's Liberal Democrats are now proposing. The party which, not so long ago, was proposing another 1p on income tax to boost education spending, now wants to cut taxes for ordinary families and pay for it by chopping £20bn from public spending. For now, the Clegg agenda is devoid of hard detail. It's all fine aspirational stuff. But it's such a flip-flop from where the party previously stood, it opens it up afresh to the old allegation that, if you've no hope of forming the government, you can offer the moon stuffed with green cheese and still not be listened to.

Tax is not something that bothers Alex Salmond and the SNP too much since, short of fiscal autonomy being implemented, they don't have to do too much of it. They make the most of their small-business bonus scheme. So I was astonished to hear, just before I went on holiday, that, despite the current economic downturn, as many as 35,000 qualifying businesses have so far failed to apply for the reductions in non-domestic rates available under the scheme.

That analysis was done by the Federation of Small Businesses. But much more damaging to SNP thinking on tax is the FSB's submission on the Salmond administration's plans to replace the council tax with a flat-rate 3p local income tax. Like every other business organsiation, it doesn't like them one little bit. FSB policy convener Andy Willox describes it as "unbelievable that the impact (of LIT) on small businesses was brushed aside" in the proposals. These are, indeed, taxing times for politicians of all stripes.