John Rentoul: The party awaits The Issue to topple Brown

The catastrophic vote against MPs' expenses reform may not be the Prime Minister's undoing, but something will be before long

She really does not want to be prime minister. That was my first thought when Jacqui Smith, the Home Secretary, voted on Thursday night to keep MPs' expenses payments. Curiously, a colleague of mine had an equal and opposite reaction. He thought that she – and Andy Burnham, Secretary of State for Culture, who also voted to keep the sty well stocked – really wanted to be prime minister.

He thought that they were trying to curry favour with the tribal class warriors of the Labour backbenches, who voted down what they saw as a plot by rich Tories to take away support for working-class representation.

Someone has misunderstood the "most sophisticated electorate in the world" – a description of the Parliamentary Labour Party attributed to Anthony Crosland – although I am not sure if it is my colleague or Smith and Burnham.

The way to become Labour leader may be to appease the tribal, Tory-hating instincts of Labour MPs, but not in such a crude way. And last week's vote was crude. There may well be good arguments against the particular reforms to the expenses rules that were proposed by the Members' Estimate Committee. They are irrelevant when set against the indefensibility of claiming expenses without receipts and using public money to buy property for profit. But what swayed Labour MPs was the sentiment expressed by Ian Austin MP, one of Gordon Brown's aides, to George Osborne, the shadow Chancellor: "It's all right for multimillionaires."

Labour MPs may not be very clever – that 146 of them voted to keep their expenses is proof enough of that – but they are not completely stupid when it comes to choosing a leader. Then, Tory-hating tribalism will take the form of studying opinion polls and deciding which candidate would be the most effective anti-Conservative weapon.

To vote in a way that is bound to be seen by almost all opinion-poll respondents as a vote for self-interest is, therefore, not the action of a serious contender for the top job.

David Cameron certainly understood the crude politics of the expenses issue. He required the shadow Cabinet to vote for reform. Although he had to endure a small back-bench revolt from the likes of Ann and Nicholas Winterton, the shameless couple who voted to keep the rules under which they claimed rent for a house they owned outright, the embarrassment was all on Brown's side.

Having voted for pay restraint, the Prime Minister left the Commons, knowing that the subsequent votes on expenses would go against what he wanted. Not only that, but Nick Brown, his deputy chief whip, would be voting against reform.

The outcome was a first-class, Michelin-star disaster for the Prime Minister. It could be almost as bad for his reputation as the abolition of the 10p tax rate. Since then, the Government's biggest problem, the $140 barrel of oil, has come from outside. But this, once again, was Brown's own fault.

At every stage, he had failed to see the problem coming; while Cameron, far more of whose MPs have been implicated in helping themselves to public money, has nimbly stayed one step ahead of the waves of public anger sweeping up the beach. Two weeks ago the Tory leader ruled out compensating MPs for the abolition of the second-homes allowance and repeated the line, "We are part of the problem and we must be part of the solution."

Brown, meanwhile, seemed to have no inkling of the damage that the expenses story could inflict. It is not as if it came out of nowhere. For months the Speaker, with Brown's tacit support, tried to block freedom of information requests for details of MPs' expenses.

When that failed, and the Derek Conway case made reform inevitable, Brown – unlike Cameron – had no plan. After 33 ministers, including four members of the Cabinet, voted against reform on Thursday, he said: "I was not happy with what happened. I am very disappointed about the vote."

Why, then, had he done nothing about it? The outcome: Labour MPs look greedy and Brown looks weak.

Just as he looks weak when it is accurately reported, as it was last week, that he has been consulting Alastair Campbell and Peter Mandelson.

The only consolation for the Prime Minister is that MPs' expenses are not "The Issue". Labour's mood is bleak at Westminster. MPs have decided that Brown is not the best person to lead them into the next election. But there are invisible props keeping him in his job. Until most of the members of "the most sophisticated electorate in the world" can answer four questions, Brown will stay. Those questions are: Who? How? What about the election? And, What is The Issue?

The first is close to being answered. Labour MPs are not wild about David Miliband but enough of them think that he will do. Alan Johnson, Secretary of State for Health, is the main alternative, but he doesn't want it and Miliband does. I was struck by the comment of my esteemed colleague Steve Richards, who interviewed the Foreign Secretary last week and said that he was "certain" that Miliband "will stand for the leadership" if Brown stood down.

There would be other candidates: probably Ed Balls, Harriet Harman and John McDonnell. But, on the basis that Labour MPs – and party members and trade unionists, who will also have a vote – will take their cue from the opinion polls, I think Miliband would win. One Labour MP tells me that he has tickets to Paraguay as an insurance against a Harman victory.

James Purnell, increasingly admired by commentators, including me, is more likely to become Miliband's Chancellor of the Exchequer.

Then there is the How? Again, it is close to being answered. "Cabinet colleagues have to fulfil their responsibilities," says one former Cabinet minister. "They have to tell him that unless he goes, they will."

The next question is harder, and not yet answered. This accounts for why, as my other esteemed colleague, Alan Watkins, writes on page 57, the move against Brown is always being put off. The voters will ask, What about us? Even if there is a Labour Party election, which there was not when Brown took over, they will complain of a lack of wider democratic legitimacy. There will be demands for an immediate general election.

MPs to whom I have spoken divide into two schools of thought. Some say play it long. "There could be a pact among the candidates that they would all say that they will go to the country in May 2010." Others suggest an election next summer, giving the new leader only a short time to set out the Government's new direction.

But no one knows what effect an internal election and a new leader would have on the opinion polls, even while they are convinced that a new leader could not be worse than Brown.

So they wait for The Issue. What to say about an election is a second-order question that partly depends on when Brown goes. And that depends on when The Issue comes along and the Parliamentary Labour Party goes into an emotional spasm.

The abolition of the 10p tax rate, as reflected in the Crewe by-election, nearly tipped them over. There were those around Brown, I understand, who thought he might be toppled that weekend. It was dangerous because Labour MPs felt that their leader had offended their principles. The vote on expenses, however much it further weakened Brown, wasn't like that: last week, they offended their leader's principles.