Politics tests character, often to destruction. The character of some ministers, their shadows and MPs of all parties has been wrecked by ­exposure of their ­expenses. How can those caught pilfering from the public purse denounce benefit fraud? How can those with state-purchased silk cushions support the cash-limited social fund that denies beds and blankets to families sleeping on bare boards? MPs with fingers in the till will blush to justify paying the unemployed £60.50 a week to live on. Nor can they rant convincingly at City greed or tax-dodgers fleeing to Guernsey.

The one character who has been tested to final destruction is Gordon Brown. The music stopped on his watch, first for the economy and now MPs' sleaze, for which the government of the day takes most blame. Labour used to lay claim to higher moral ground, while the right always said greed was the motor of growth. When he first talked of his moral compass, Brown should have cleaned up party funding, MPs' expenses and honours – and linked these reforms with curbs on the power that money breathes over the nation's affairs. The expenses mess would not be fatal if the prime minister were upright and strong. But Labour is already ­dangling over a cliff, and this affair prises its fingers off the edge.

It's all over for Brown and Labour. The abyss awaits. As long as he remains leader, there is nothing that wretched Labour candidates can plausibly say on the doorstep at next month's European elections. They are struck dumb. Why should people vote for them? The horse manure bought on expenses is garnish for a decomposing government. The heart of the matter is the economy, and Brown's responsibility for the bubble years. He personally is to blame for Labour's failure to ensure that ordinary people on median incomes and poor people at the bottom received a bigger share in national growth: it turns out that they fell back and only the wealthy prospered. Labour made the rich richer and the poor poorer: growth for the few, not the many.

That is a failure so fundamental to Labour's purpose that the party can't go into the next election led by the man responsible. His other failings as leader pale beside this one monumental fact. While he is there, Labour cannot claim "fairness" or "social justice", so what is left to say? What is Labour's offer?

Gordon Brown has been tested and found in want of almost every attribute a leader needs. Squalid dealings by his poisonous inner circle were exposed to the light of day; yet at the same time he lacks a leader's necessary political cunning. Many hoped that the end of the rivalry with Blair would see Brown cast off his myrmidons. He didn't. In the tussle between his better and his worse selves, too often the lesser man won.

That he was no great public orator or warm telegenic talker would never have mattered had he gained a reputation as a gruff, unspun man of honour, vision and purpose. I thought it an asset after Blair's glibness and Cameron's suavity. It wasn't the medium that did for him, but the message. There wasn't one. What was Labour for?

He may be the best-read prime minister in decades, but his learning seems to hamper instead of illuminate his path. His indecision is legendary, every department awaiting answers that linger on his desk for months as he agonises sleepless but indecisive into the early hours. But then the decisions he takes are too often tactical, not purposeful or strategic. Trident, the third runway or post office privatisation are mere positioning in some illusory business-pleasing ploy, their long-term damage far outweighing one day's headlines.

Blair people warned of Brown's dark side, his rages, obstinacy and inflexibility. Labour MPs who voted him in ­unopposed hoped he would grow in stature. They needed to believe the best of him as there was no alternative. Any serious attempt to stop him would have led to an internal feud of such ferocity it would have shipwrecked the government. Besides, back then the economic boom years were his crowning laurels.

I was among those looking for the best in him, celebrating his undoubted concern for Africa, foreign aid and child poverty – but no one can know a leader's mettle until too late. His leadership of the G20 championed a measure of Keynesianism to counter the worst effects of the crash. But an essentially neoliberal ideology coupled with timidity prevents him taking this once-only chance to reform the City, demand more of bankers and separate high street from casino banking. Despite the crash, he harbours the same old reverence for, or fear of, the money-men who wrought this global mayhem.

The morning after the 4 June ­election a majority deputation from the ­cabinet, bearing a long list of MPs' names, should knock on the door of No 10 to tell him his number's up. Plot it now, do it fast. The Tories are lethal with their failed leaders: Labour MPs facing annihilation must find the bottle. There is nothing to lose. Once the credit crunch began, I thought assassination might make ­matters worse, precipitating a ­downward spiral from which Labour could fall into total collapse – Brown at least had the gravitas of experience. But Labour now faces an imminent collapse anyway, with Brown hitting polling depths below Michael Foot's, lower than for 70 years.

There is all the difference between losing by a few points and crashing out so badly it takes ­another three elections to ­recover. The one person around whom the party could gather speedily would be Alan Johnson. It's nonsense that another unopposed leadership would mean disaster: a general election is coming soon enough. Orphan boy, genial postman, self-made, clever but modest, he has the grace and charm to match his perfect backstory. He was always the one the Cameroons feared. His political talents turned the NHS from a danger with closures and denials of drugs into an asset for Labour. Good to work with, good in public, he inspires considerable admiration. This time I will not say I know he would be a good leader – that's unknowable until too late. I doubt that he can win for Labour. But, goodness knows, Cameron is still there for the taking.

The only question now is whether Labour ministers and MPs are so shell-shocked by the last year and so shamed by their expenses that they lack the will to live. Ordinary party members, you valiant few, get up and tell your MPs that Gordon Brown must go.