You know it is over when they laugh like that. Most of the noise in the Chamber of the House of Commons is uncouth and childish; it is also often manufactured, designed to harass and demoralise the other side. But when Gordon Brown headed for the exit after Prime Minister's Questions, and had to turn round, realising that he was supposed to making a statement about Afghanistan and Pakistan, the laughter from the opposition benches – and from some on the Government side – was genuine. It was because it was genuine that it was so cruel.

The Prime Minister has lost his way. He has lost his place in the script. You know it is over when Nick Clegg cuts it as a figure of moral authority, and Brown is reduced to making up numbers such as £1.4bn as the cost of allowing all 36,000 Gurkhas the right to live here. Even if it did come to £1.4bn, which is doubtful, why should we draw the line there, after £175bn of fiscal stimulus, on the one immigration issue on which even Empire loyalists are on the liberal side of the argument?

Brown has lost the argument about the Gurkhas so comprehensively that David Cameron did not even need to rehearse his Mr Angry act. He did Mr Bipartisan instead, congratulating Clegg for setting the pace on the issue. It was a smart bit of tactical cross-party generosity that diminished Brown further.

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No, you know it is over when BBC journalists start interviewing each other about how much the Prime Minister's "authority" has been reduced. They were at it this week over the withdrawal of Brown's plan to reform MPs' expenses. They used to do it to Tony Blair when he was at bay over the cash for honours investigation, but they didn't laugh at him. Blair was spared the added humiliation of YouTube ridicule.

You know it is over when black is reported as white. When everything is fitted to the template of retreat, disarray and incompetence. Just a small example from this week: David Blunkett, the former Home Secretary, repeated his ingenious plan to make identity cards more palatable by making it compulsory for everyone to have a passport. This was reported as Blunkett, "the father of identity cards", calling for the scheme to be scrapped.

We have been here before. In fact, we have been here twice in living memory. James Callaghan and John Major seemed similarly doomed, especially in retrospect, as they limped towards their conclusions – in Callaghan's case 30 years ago this Sunday. But Callaghan retained his dignity and not even Major cut so miserable a figure as Brown does now.

Those were "sea change" moments. Although we should be wary of granting sly Sunny Jim the excuse that he was up against the inevitable: his observation that there are times when it "does not matter what you say or what you do" concealed his regret that he had not gone to the country the previous autumn when he could conceivably have held on against Margaret Thatcher.

There is always something you can do. Major probably should have stood down, as he briefly consulted close colleagues about doing, immediately after the collapse of his ERM policy. Michael Heseltine might have swashed some buckle and gone to the country. Brown could stand down now, as even Paul Routledge, his formerly sympathetic biographer, suggested last week, and let someone else try to limit the Conservative gains at the election. Routledge and I, who have not agreed on much for a decade and a half, agree that Alan Johnson is Labour's best hope.

I think it is worth a try, from a Labour point of view, even if it succeeds only in cutting the Conservative majority – and the current state of the betting markets points to a majority for David Cameron of 62. Some Labour people don't see the point. Their unspoken belief is that the next election is a write-off, so the party might as well get used to a long period in the wilderness.

This is sea-change thinking, otherwise known as giving up. My view is that it is worse than unwise; it is a mistake. It is based on a fallacy, namely that Labour will have been in office for 13 years and before that the Tories were in for 18, so whoever wins the election will occupy Downing Street for a long stretch.

On the contrary, it seems more likely that we really are back to the Seventies, in the sense of alternating parties in government and inconclusive elections. To be brutal, the next election is a good one to lose. The state of the public finances is such that, if the Conservatives win, as Cameron told his party's spring forum in Cheltenham on Sunday, they will be "in an age when we're asking people to put up with tax rises and spending cuts to pay for Labour's debt crisis". Note that he said tax rises and spending cuts, because I'm not sure that his audience in the hall heard him.

When the Tory members find out – after, say, George Osborne's third tax-raising Budget – they are not going to be pleased. Nor will the British voters be. Cameron can talk the New Labour talk of difficult choices but when it falls to him actually to make some we are not going to like it. So it is quite possible that, if the Conservatives win next year's election, they will be unpopular quite quickly. It is not as if the electorate are even under any illusion, as they were when Blair came in with a 93 per cent approval rating, that Cameron represents a "new politics".

Provided the Labour Party does not fall apart – and it is not divided by anything like the quasi-Marxism that afflicted it in 1981 or the issue of Europe that split the Tories after 1992 – general elections promise to be competitive again. So things are bad for Brown, but this is not a sea change. There is certainly nothing inevitable about a long period of Conservative government. Indeed, we could be heading for a long period of hung parliaments. Yesterday was the right day for Nick Clegg to shine.

John Rentoul is chief political commentator for The Independent on Sunday. You can read his blog at http://johnrentoul.independentminds.livejournal.com/