It is the budget again, and time for another supposed attack on the middle classes from this radical coalition government. If you believe the Daily Mail, you would think that the coalition is in the pay of leftwing union leaders because it is so determined to tax "hard-working, aspirational, self-reliant middle Britain".

Two former Tory chancellors, Nigel Lawson and Norman Lamont, have made the case for alleviating what is billed as a disincentive to work, risk-taking and prosperity. They want the government to take more people out of the 40% income tax band rather than increasing the tax-free personal allowance (which stops the low paid from paying income tax at all).

When Lawson introduced 40 pence in the pound in 1988 as the top rate of income tax, 1.7 million people paid it. Now it starts at an annual income of £41,866, and catches 4.4 million people. Many have only recently started paying the 40p tax rate, as one of the coalition's surreptitious tax increases has been to cut the starting point.

However, don't cry for middle Britain. There are more than 30 million people in work across the country. Just 15% of taxpayers earn enough to pay the 40% rate, and they do not pay the higher rate on their entire income but only on the extra income above that starting level. It is absurd to describe anyone in that top group – barely one in seven of the tax-paying population – as part of middle Britain.

What about the 85% of income earners who are not paying the 40% rate? They include the vast majority of the middle class, let alone blue collar workers. The Oxford English Dictionary says that middle is "used of that member of a group or sequence, or that part of a whole, which has the same number of members or parts on each side of it". On this definition, the income of someone exactly half way between the bottom earner and the top was precisely £26,884 a year in April last year.

So middle Britain is nowhere near being caught by 40% income tax rates, and anybody who suggests that it is should be frogmarched to the Plain English Campaign for a refresher in reading, writing and arithmetic.

However, there are a lot of people who work in the Westminster bubble who pay 40% and are not as typical as they like to think. Even the editor of the Daily Mail, Paul Dacre, regards himself as a champion of the middle class, although his annual pay was some £1.85m last year.

We can safely dismiss the argument – a little sign of desperation here from the chancellor, reportedly faced with hostile Tory backbenchers – that people like paying the higher rate because it is a sign of their success. The same argument would have us back at Denis Healey's 98% on unearned income in no time at all.

There is, of course, a respectable case for any tax cut: it may boost the incentive to work more; and it boosts spending, particularly if aimed at low earners who can afford to save less. The economic evidence also suggests that it is the poorest earners, not the highest earners, who are most discouraged by high tax rates on extra income. Money means more to them because they have less of it. Before they take a job or work more hours, the low paid count their bus fares and look at what they will lose in income tax, national insurance contributions and in-work benefits.

The low paid also suffer far higher rates of tax and benefit withdrawal on extra income than 40%: many face 70% effective tax rates on each extra pound they have (taking account of income tax, national insurance and the loss of income-related benefits). That is why it is so much more important to lift the low paid out of tax altogether than to provide concessions to upmarket folk who are deluded enough to think that they are merely middling.

There is no easy answer, but the Liberal Democrat policy of increasing the allowance you can earn without paying any income tax helps to provide the poor with paths out of poverty through work. (True, it helps richer taxpayers too.) The government could lift low incomes more directly through higher benefits, but that would be at the cost of state dependence and, worse, work disincentives.

It is crazy that we legislate for a minimum wage – now £6.31 an hour but set to rise to £6.50 in October – and then tax people on it. Yet anyone on the adult minimum wage throughout the year will earn £13,000 and will pay both income tax and national insurance worth in excess of £1,200. Both tax and national insurance starting points should surely be raised above the minimum wage.

What will the chancellor do on Wednesday? George Osborne has riled his Liberal Democrat colleagues by trying to take the credit for the increase (so far) in the starting point for income tax from £6,475 to £10,000, a notably popular policy that has lifted about 2 million people out of income tax altogether. But it made no appearance in the Conservative manifesto – Osborne famously panicked Gordon Brown into postponing the election by promising inheritance tax cuts – but was the first priority for the Liberal Democrats.

Osborne's attempt to grab the credit for this is not going well, judging by the Ipsos-Mori poll last week that showed 45% rightly thought the Liberal Democratss deserved most credit (compared with 33% for the Conservatives). If Osborne wants to steal more of the credit he is going to have to go further than the Liberal Democrat manifesto, however hard the Daily Mail and the ghosts of chancellors past may find it.