Politics now seems like those books with titles along the lines of Why Everything Is Rubbish – only they don't use "rubbish"; they use crude and childish words that some people find funny. There is even a book called How Not to Talk Like an Arse. In which the first advice should be: don't use words such as "arse".

According to the opinion polls, all political parties are rubbish and all their leaders are – well, fill in your own crude and childish insult. One telling series is our ComRes ratings of the party leaders. Since the election, we have asked people if they agree or disagree that David Cameron, Nick Clegg and Ed Miliband are turning out to be good at their jobs. From the start, their scores were all negative – that is, more people disagreed than agreed. Since then, they have all got worse, although the order has been unchanged. Miliband is more unpopular than Cameron, and Clegg is more unpopular still.

Cameron, as the least unpopular choice at the last election, managed to win without winning, and he is still the least unpopular leader. But, in the past few weeks, the margin by which he is deemed to be less useless than the other two has narrowed. Last month's Budget was a dose of polonium, with a half-life of several weeks, to Cameron's personal rating and the Tory party's opinion-poll standing. The damage to Tory internal organs has been shocking, and the injury to George Osborne's reputation quite possibly irrecoverable.

Hindsight is a wonderful thing, and a source of many wise and far-seeing judgements, but it was not needed in this case. All the disasters in the Budget speech were spotted by journalists, accountants, the Labour Party and several coalition MPs while the Chancellor was on his feet. The pasty and granny taxes blew up as soon as he sat down. The capping of tax relief for charitable donations, although slower to become a media crisis for the Government, was also spotted at the time.

On the day, it seemed that the cut in the top rate of income tax, which Osborne did quite deliberately, was his most serious mistake. So it may prove in the long run, but in the meantime the string of disasters that no one in the Government seems to have foreseen may cause more trouble.

The more the Budget unravels, the more we are entitled to ask, what did they think they were doing? How did the Quad – Cameron, Osborne, Clegg and Danny Alexander – not see how VAT on hot takeaways, freezing pensioners' tax allowances and limiting charitable tax relief would be reported? This matters not because we think, as with the "tax cut for the rich", that their values are wrong, but because we think that they are incompetent. It causes us to wonder about the chiefs-to-indians ratio in the coalition's inner workings. Whose job was it to bombproof the Budget?

The charity tax bungle is perhaps the hardest to explain. We could deduce this from news clips of the Prime Minister, asked questions in Indonesia about Budget small print and needing two minutes for one answer. In which he said he wants to encourage philanthropy in the same sentence as saying that the tax relief has to be cut because it is being abused. This took longer to say than it should have done because he carefully avoided blaming Nick Clegg. That was very polite and coalition-minded of him, since it was the Deputy Prime Minister's bright idea of a "tycoon tax" that lies behind this bungle.

The tycoon tax – a minimum rate of income tax that everyone should pay, regardless of the reliefs and allowances they claim – gave Treasury officials an opening to try to limit the relief on charitable donations. As it happens, I agree with Janet Street-Porter, who argues today that the general taxpayer should not subsidise through tax relief the charitable priorities of the rich. Indeed, Clegg himself makes this argument in our interview with him today. But that is not the argument that Cameron or Osborne want to make, which is why they fall back on the unconvincing story about the tax relief being abused, for which the evidence is thin. Besides, if the tax relief is being abused, surely the abuse should be tackled or the relief abolished completely? Limiting abuse to 25 per cent of someone's income makes no sense.

Yet Clegg has so far managed to evade responsibility for the "charity tax" disaster. Strange how the requirements of coalition, so terrible for Clegg's popularity in the first two years, are now hurting the Conservatives. Just as the Tories benefited from the coalition having only one head of government at the talks in Brussels, when Cameron said no to the fiscal pact, so the Tories suffer from the coalition having only one Chancellor who presents a Budget. All the unpopular bits of the Budget, including the ones caused by the distractions and demands of the Liberal Democrats, are blamed on Osborne and his party.

Thus the three parties go on, locked in a struggle to be less unpopular than each other. This kind of negative politics reaches an interim conclusion in next month's local elections. For many London voters in particular, and especially habitual Labour ones, the mayoral election presents a difficult choice between two evils.

However, and the other reason why I dislike those Why Is Everything So Rubbish? books, neither life nor politics is more rubbish than it used to be. Certainly, it seems unusual that all three main party leaders should be so unpopular. But, actually, it isn't. Ipsos-MORI has opinion polls going back to March 1977, when 64 per cent were dissatisfied with the Callaghan government and only 27 per cent satisfied, yet 51 per cent were dissatisfied with Margaret Thatcher, the leader of the opposition, and only 36 per cent satisfied. For most of the next 35 years, voters were more dissatisfied than satisfied with all governments and all party leaders.