LONDON — I knew Jeremy Corbyn before he was famous. The Labour Society at my local university, Queen Mary, invited him to a debate two years ago. The motion was: “This House Believes Tony Blair Saved The Labour Party.” You know which side he was on. I was on the other. There was a show of hands at the start, which was about two-to-one against the motion, and again at the end, which was more like 10-to-one. Talk about a premonition.

I learned then what many more people now know. Corbyn was personally pleasant, polite and unassuming. In debate he was effective. He rehearsed the far-left verities. They sounded more like slogans to me, each one plausible, but it was hard to challenge any of them because he just moved to the next. Inequality increased during Blair’s government, he said — no it didn’t, actually, said I; Blair failed to challenge the interests of the rich — but he promoted the interests of the poor; he was deferential to Rupert Murdoch — he tried to ensure a fair hearing for Labour in the media….

The Queen Mary students liked Corbyn’s plain style. It wasn’t oratory, but it seemed straightforward. Above all, it went with the grain of what “everybody knows”: That New Labour was all about spin and war and recession. I tried to make them think about the complexities and compromises of a largely successful government.

You are right: I am not much of a debater.

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Now a similar thing is happening on a national scale in the election for the leadership of Britain’s Labour Party, which was routed in May in the general election. Corbyn, the token candidate included “to widen the debate,” is now winning it. He is sweeping up the votes of Labour members and supporters signing up to take part in the leadership election by the tens of thousands.

"He seems [the only Labour] candidate who believes anything, right or wrong” — Rupert Murdoch

This week Murdoch himself said he thought Corbyn would win — he “seems [the] only candidate who believes anything, right or wrong” — and one betting company paid out to gamblers as if he already had. Unbelievably, most of the MPs who nominated him don’t actually want him to be leader. That means that the anomaly of Ed Miliband’s election, against the wishes of just more than half of Labour MPs, is likely to be followed by the nightmare for Labour in which a mere 7 percent of its MPs positively support their leader.

Many of them, we discovered this week, have never even spoken to him. When Corbyn stands up as Leader of the Opposition at Prime Minister’s Questions on September 16, he will not be greeted by full-throated cheers from the benches behind him. Most Labour MPs, torn between insincerity and dissent, will simply stay away. Most Conservative MPs, on the other hand, will be torn between gloating and disbelief. Both sides of the House, and the galleries above, will be asking, How did he get here?

The answer to that question is that it was a catalog of accidents and mistakes. Corbyn is not Chauncey Gardiner, the simple-minded gardener who accidentally becomes President of the U.S. in the film “Being There.” For a start, he is not an idiot. But his candidacy was accidental in that it was simply his turn, as a long-serving member of the small group of hard-left Labour MPs. Diane Abbott ran for leader five years ago, coming fifth out of five. John McDonnell and Michael Meacher tried to run in 2007, when Blair stood down, but neither could muster the required number of nominations. Corbyn was the only one left with any national profile — apart from Dennis Skinner, who is 83. So Corbyn, a mere 66, got to go.

Then there were the mistakes: not just by those MPs who nominated him to please their activists with no intention of voting for him, but by the other three candidates in failing to inspire. The last mistake might have been the decision by Harriet Harman, Labour’s acting leader, to order MPs to abstain on the government’s welfare bill that would cut tax credits for the working poor. The parliamentary tactics were too subtle for Labour members and supporters: They cheered on Corbyn, who defied instructions and voted against.

Occasionally, the accidental candidate has seemed bemused by the attention. My colleague Jane Merrick reported from a green rally during the campaign. While he was waiting to be introduced, Corbyn, “in what appears to be a momentary lapse in concentration, puts his hand into his trouser pocket, pulls out a £5 note, stares at it with a look of surprise that is neither excited nor disappointed, and puts it back.” It was at that event that he said he wanted “houses with gardens for everyone,” and added that “anyone who wants to be a beekeeper should be a beekeeper.”

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Like Chauncey Gardiner, he is a candidate in whom his supporters can see what they want to see: In his case, honesty, principle and unapologetic anti-Toryism. Indeed, he has been consistent to a fault in his career. It is one of the worst things about him. He has opposed every military action in which his country has engaged. He was opposed even to wars that most people thought were justified, such as the rescue of the Kosovo people from Slobodan Milošević's attempted genocide. This so incensed fellow left-winger Clare Short, then a Labour government minister, that she said he was a disgrace to the Labour Party and “like those who appeased Hitler.”

In fact, Corbyn made his name because he opposed Britain’s part in the U.S.-led action in Afghanistan. This was widely supported around the world as self-defense under the U.N. Charter at the time, but Corbyn became chair of a new organization called the Stop the War Coalition, which, 18 months later, found itself at the center of mass protest against the invasion of Iraq.

Elections — including party leadership elections — are painted in primary colors. Never mind that I think one of his reasons for opposing the overthrow of Saddam Hussein was mistaken, namely that it was being undertaken by America. He put himself vividly on one side of the “war” versus “no war” divide. (There is a strong strain of anti-Americanism in Labour opinion and British public opinion generally, in any case.)

Corbyn is able to maintain clear and simple positions because he has never had to take responsibility for anything.

The consistency of his opinions since 1983 ought to alarm people. What normal person holds the same views not just on general principles but on specific policies from the age of 34 until 66? Yet he still adheres to everything in the Labour manifesto on which he was first elected — apart from a classic, towering, equivocating politician’s fudge on the question of whether we should be in or out of the EU (he wants a people’s assembly to advise him how to vote in David Cameron’s referendum). He wants to renationalize industries sold off by Conservative governments, probably even including “the building materials industry” and “film distribution” (echoing the 1983 manifesto). And he wants to give up Britain’s nuclear weapons and to pull out of NATO.

Corbyn is able to maintain clear and simple positions because he has never had to take responsibility for anything. He has never been a minister or held a front-bench post in which he spoke on behalf of the party. In many ways, he has hardly been a member of the Labour Party for years, voting against the party line in Parliament more than any other MP.

But what I think disqualifies him from the leadership is precisely what attracts his new supporters. Andy Burnham, for instance, is running as the candidate from “outside the Westminster bubble,” but he has been a political adviser, mainstream politician and minister all his life. Corbyn is just as much a creature of Westminster, of course, but always as the rebel. If you want to vote against politics as usual, Corbyn is your candidate. The more the party bigfeet tell you it would be a bad idea and would lose Labour the next general election, the more you want to do it, just because it will cause a sensation — and anyway, Corbyn’s chances in 2020 can’t be much worse than those of the others, can they?

"His policies are nowhere near as popular with the wider electorate as his supporters think they are."

Well yes, they can. But it is too late for that now. Corbyn’s supporters are just going to have to find that out for themselves, either the hard way, by losing at least one more election, or the slightly less hard way, by reading the opinion polls and taking evasive action.

Before then, their new leader may enjoy a honeymoon with public opinion. Corbyn is certainly a break from the “cult of the young man” in politics about which Tessa Jowell, likely to be Labour candidate for London mayor, spoke at the weekend. But his policies are nowhere near as popular with the wider electorate as his supporters think they are.

If Corbyn were a greater personality, he might be able to exploit his moment. But he is no Michael Foot, an intellectual and former Cabinet minister. He is not even a Tony Benn — and Benn, let us remember, could not even get elected as the party’s deputy leader in 1981, after much of the centrist wing of the party broke away to form the Social Democratic Party. They are choosing Corbyn for his anti-charisma. They think it is refreshing now. They don’t think it matters that he responds reluctantly and with flashes of bad temper when challenged about some of his associations. He has no qualities of leadership except those that are projected onto him by others. And he has no party in the House of Commons.

His election as leader would be the most extraordinary act of self-harm ever committed by a major British party, and will scar British politics for decades.

John Rentoul is chief political commentator for The Independent on Sunday and a biographer of Tony Blair.