Something unexpected happened in the House of Commons last week, only no one noticed because we were laughing at the Prime Minister. He got his parties muddled and said: "It is the Liberal Party that wants to cut public expenditure, not the Cons – not the Labour Party." At this point Hansard recorded an "interruption". How funny it was that Gordon Brown had tripped over his own ridiculous attempt to accuse the opposition parties of wanting "10 per cent cuts" in public spending.
No one noticed that the Prime Minister had expressed a surprising truth. Who is the most convincing axe-man? Which party is most serious about deep and specific cuts in public spending? It is not Cave-Man Dave and his Prehistoric Thatcherites; it is Nick Clegg and the nice "Liberal Party", as Brown insists on calling them – a completely pointless and unwounding supposed insult.
Because there was such a hooting at the Prime Minister fluffing his lines, we missed the importance of what came next. Clegg told the Prime Minister: "Today, new figures from the EU have been published that show that we have the largest underlying deficit anywhere in Europe. Why does he not admit that balancing the nation's books will take big, difficult, long-term decisions?... We are setting out what needs to happen – unlike him, and unlike the Leader of the Opposition – on Trident, on baby bonds, and on tax credits for high-income families."
David Cameron, accused by Brown of being hell-bent on cutting spending, has not been anything like as specific. Gordon Brown, whose Chancellor set out the spending plans which imply the very "10 per cent cuts" which the Tories are accused of wanting, has been even less specific about how the Government's budget is going to be brought back into balance.
Cameron has been preparing the ground with general declarations of "fiscal responsibility". Brown and his Cabinet have been talking about "difficult choices". Ben Bradshaw, the new Culture Secretary, talks of them in his interview with this newspaper today. But it is Nick Clegg and the Liberal Democrats that have actually been making them.
And they are difficult. Cancel Trident? Suddenly Conservatives who are happy talking in general terms about shrinking the size of the state decide it is essential. And New Labourites who take their cue from Bill Clinton decide that he was a "national security" Democrat more than he was a "balanced budget" one.
There has been a macho tendency among members of the shadow cabinet recently to declare that the Cameron government will be very unpopular very quickly because it will have to impose the deep cuts set out in Labour's plans. But where will the axe fall? Except that NHS spending has been protected, along with the international development budget (which is tiny), we don't know.
All we know is that George Osborne, the shadow Chancellor, will convene a "two-day emergency cabinet session soon after the Tories gain power", according to "shadow cabinet sources" quoted last week. I suspect Cameron and Osborne's plans are a little more advanced than that – not least because they appreciate some of the complexities of which the "shadow cabinet sources" seem unaware.
One is that Cameron is likely to be an unusually weak prime minister. He will continue the trend of the past 12 years. For all that Tony Blair was attacked as an elective dictator, he ceded more power than any of his predecessors, handing the power to set interest rates to the Bank of England and deferring in more and more areas to advice from officials and independent experts. Cameron intends to give up even more scope for making economic decisions to an independent Office of Budget Responsibility, which will be charged with balancing the government's books. So even if Cameron has specified few of the cuts he will make, he is committed to creating the mechanism that will force him to make them.
Not only that, but a Conservative government is likely to have a small majority or no majority at all. I assume that the Labour Party will recover from its current trough of unpopularity, by which I mean that it replaces Brown with Alan Johnson.
That is why Clegg's intervention was so significant. Until now, I had assumed that the Lib Dems would be closer to Labour in a hung parliament on the core issues of tax and spending. Indeed, Clegg told me last week that Brown's "defence of the indefensible" was obscuring the "common ground" between the two parties. "We must not make cuts that imperil the life chances of the next generation," he said, making the case for protecting spending on young children.
But Clegg's larger point is that the public mood has changed. He has been shaken by the party's private polling, which found a striking number of voters who say, when asked who is to blame for the economic crisis: "We all are." This adds texture to the numbers in a YouGov poll last week, which found 79 per cent accepted the need for "less public spending" in order to balance the nation's finances.
In this climate, Brown may have misjudged his dividing line. When he accuses his opponents of wanting 10 per cent cuts in public spending, the voters might say, "Yes, please." Clegg said that the vast overhang of the national debt – approaching the levels accumulated during the Second World War – "will dominate politics possibly for as long as I'm active in it".
So while he is not in favour of cuts in public spending this year – contrary to Brown's attempt to pretend that he is – he is likely to fight the election on a more explicit programme of public spending cuts than the Conservatives. And it may be that a Cameron government, even a minority administration, will not only be required by its own watchdog to make deep cuts in public spending, and be able to do so with Liberal Democrat support, but it may not be unpopular if it succeeds.
John Rentoul's blog is at www.independent.co.uk/jrentoul