There is a case for saying that Tony Benn was one of the most impressive Labour Party figures of the 20th century. Tireless, always eloquent, a thorn in the flesh for dissemblers, careerists and warmongers, an inspiration for a great many socialists: he was all that and more.

Somehow, equally, the grand old man was all that and less. By the end, he knew as much himself. In later life, placed under political house arrest as a national treasure - not that he complained - he understood the praise that came his way from former enemies. There was plenty of it.

They would talk about the great parliamentarian, the indefatigable diarist, or the ever-reliable protester as an ornament of British life. They would hail him as heir to a long tradition of English dissenting radicalism. They did so because he caused them no trouble worth the name. For the last 30 years of his life Mr Benn was, in his own description, "harmless". Yesterday, with the announcement of his death, the glib praise echoed again across the airwaves.

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In fact, one brief period in the early 1980s aside, he was never more popular than when he was least dangerous. Even when he challenged Denis Healey for the right to become Michael Foot's deputy in 1981, the only real risk Mr Benn ever posed was to a Labour Party leadership bent on its long march to middle-of-the-road orthodoxy and blandness. He caused havoc in his own ranks while real enemies looked on contentedly.

Some would excuse him for that and ask what, in the circumstances, a socialist was supposed to do. In one version, the "Bennite" schemes for Labour came with impeccable democratic credentials. Here was a chance to hold the leadership to the manifesto, to impose the will of the members and the national executive - Mr Benn's power base, by no accident - on the parliamentarians. Set in a favourable light, the entire campaign was an exercise in attempting to secure representative democracy.

The battle had been a long time coming. The tensions between left and right had caused tumults within Labour's broad church periodically since its founding. Though Mr Benn was no Marxist, and scarcely a deep (or consistent) political thinker, he managed to embody an argument. Was Labour actually a socialist party? Ought it to be such a party? If so, what did the Bennites mean by socialism after they had delivered themselves of melodrama on the economic commanding heights, nationalisation, capitalism and the siege economy?

A simple but consistent criticism of Mr Benn was that he never got around to describing what a socialist society would actually be like. His critique of the status quo became resonant as globalisation engulfed the world and Britain followed the United States into war after war. When the world's financial systems came close to collapse in 2007-2008 the erstwhile 1960s technocrat with few ideological pretensions could claim to have been vindicated. But he was always better at describing what he opposed than in spelling out what he proposed.

Mr Benn lost to Mr Healey in the autumn of 1981 thanks to the electoral college he had championed. He then handed a gift to Neil Kinnock, Mr Foot's successor, thanks to a dalliance with Militant entryists and anyone else on whom Mr Benn bestowed the title "good socialist". But the problem was less the issue of policies - for some of those were worth the argument - than personality. The left's aspirant leader claimed to disdain all talk of that, but his personality was the conundrum at the heart of his career.

In a 1986 book entitled Loyalists and Losers, Michael Foot wrote dismissively that Mr Benn had fallen out with almost every colleague he ever had to work with. Mr Benn was liable to retort that his competitor for the affections of socialists - these comrades were antagonists for decades - was "a prisoner of the right". There was, nevertheless, truth in the charge that behind all the talk of solidarity sat an over-sized ego and a politician who was mistrusted as often as he was revered. Mr Benn talked a lot because, it seems, he never once doubted that his words were worth hearing.

Yesterday, Ed Miliband was offering up pieties. Mr Benn, said Labour's leader, "will be remembered as a champion of the powerless, a great parliamentarian and a conviction politician ... whether you agreed with him or disagreed with him, everyone knew where he stood and what he stood for". That last statement isn't precisely true: Mr Benn was as inconsistent as any politician. What is true is that modern Labour in no sense resembles the kind of party he attempted to create, and that socialism is less well represented in Britain now than when he was active politically.

The conventional wisdom has it that the Bennites "paved the way for Thatcherism" by allowing voters a good look at the unappealing face of the left in the aftermath of the winter of discontent. It makes for a handy cliché. In reality, Labour's travails at the end of the 1970s and throughout the 1980s had as much to do with the disgruntled right-wingers who turned years of plotting into the Social Democratic Party. Mr Benn might have been a handy hate figure, but the SDP ate into the Labour vote.

Yet the Bennites failed: so much is self-evident. It was not in Mr Benn's nature ever to admit that he deserved any blame for that, or for the consequences. One of those is the party Mr Miliband attempts to lead. It in turn exists because of the emergence of Blairism, that peculiar example of, if you like, entryism from within. Mr Benn and the party upheavals in which he tried to claim Labour for his version of socialism were New Labour's perfect excuse. Had not the Bennites (as opposed to the SDP) rendered the party "unelectable" in the Eighties? And didn't this make Tony Blair "inevitable"?

In numerous statements down the years Mr Benn seemed oblivious to most of this. All were out of step but him. Anyone who failed to agree with his "analysis" was guilty of betrayal. Only if the party listened to the wisdom of an erstwhile minor minister could Labour, and therefore socialism, be redeemed. Amid all his supposed radicalism, nevertheless, he remained a devoted Westminster parliamentarian of the old sort and a devoted fan of a performance named Tony Benn.

As Thatcherism emerged, Labour needed a leader from the left: that seems as true now as it did during the 1980s. Mr Benn was never that lost leader. By the time Mr Foot was given the chance, pressured on all sides, the job had become impossible. Even then, Mr Benn was not susceptible to calls for loyalty. The result was a party determined to become electable and prepared, in the end, to destroy its creed in an effort to win the prize. Blairism was its reward.

Mr Benn's admirers don't often describe his legacy in those terms. The left's habit of romanticising failures won't allow it. But his personality, not his policies, ruined Tony Benn's career. An irony is not much of an epitaph.