This is Danny Finkelstein's explanation in today's Times. It's worth quoting at some length. Apologies to Mr Murdoch for any copyright issues.
The basis of (Britain's) prosperity, liberty, tolerance and stability is our liberal market economy founded upon property rights and the rule of law.
The success of this economy is the most important thing to safeguard because of all that flows from it. I believe passionately in decent public services and a welfare state, but I believe equally passionately that they depend upon a thriving economy. . . those who want to replace capitalism with something else haven't a clue what they actually mean when they express that sentiment. Nor can they point to a single successful alternative to capitalism that exists anywhere in the world. While I wait for Russell Brand to think of one, I will remain calm about capitalism's imminent demise.
I do however believe that Ed Miliband and (the SNP) are hoping to ride to power on the back of two linked ideas about the market economy that are profoundly dangerous.
The first idea is that the limits of public spending are only set by the limits of our goodwill and compassion. This idea has formed the core of every Labour campaign in my lifetime and been the downfall of every Labour government. . . . In every election since it was founded in 1948, Labour has argued that there are just hours left to save (the NHS) from destruction by the horrible Tories. . . Yet since 1948 there have been 40 years of Conservative government and the NHS is still here. If the Tories are working on destroying the NHS they are certainly taking their time about it.
The idea that the NHS is about to be subjected, or has been subjected to some horrible right wing punishment is a childish way of escaping the obvious problem - that what prevents us from being able to buy every medical treatment we want for every person all the time is that there isn't enough money to do that. And trying to ignore that problem across every public service and in welfare means we end up with huge borrowing. That undermines the future of the welfare state with much greater certainty than any Tory moral failing. That is what Labour did last time.
"I'm afraid there is no money". Why not carve that on an 8ft stone, Ed, and stick it in your garden?
The second dangerous idea about the market economy is that the key to making people richer is to keep taking more from the very wealthy and from business. It is not that this notion is completely wrong. Of course we need progressive taxes, of course businesses should show a responsible attitude to employees and of course the law plays a role in making that happen.
Yet in the end, believing that can tax ourselves to prosperity is a delusion. It assumes people will come and bring their businesses here in order to pay more tax. It assumes no one will compete with our goods and services as we raise wages and regulation.
Just like borrowing too much, it can work for a time, but it can't work for a long time. In the end, the only thing that can sustain our welfare state and pay people decent wages is to make things and sell things and keep getting better at both of those.
We can't spend what we don't have. We can't have what we don't make.
There is plenty that can be said against David Cameron's government. I know that. But ultimately I think as prime minister he has the firmest grip on this essential point. The firmest grip on reality.
The last five years Britain has done something very important. It has shown the will to drive down current spending costs and get the economy growing again. The case for David Cameron is that we will only go on doing that if he is returned to office.
Election day is tomorrow.
