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If there is one thing everybody following British politics over the last month can agree on it is that Jeremy Corbyn is unelectable.

“Nobody”, former Labour cabinet minister Peter Hain wrote in the Guardian last month, “imagines he could be prime minister.” The unelectability of the MP for Islington North is like the British weather: we might not like it but there’s nothing we can do about it. Frustratingly, this is a common belief among Corbyn supporters too.

When it comes to progressive social change it seems lots of people adhere to what Red Pepper co-editor Hilary Wainwright describes as “the highly conservative assumption that what is possible is reducible to choices between what exists or has historically existed.”

However, we don’t need to imagine a wholly new future to challenge the political elite’s framing of Corbyn as unelectable. Rather, all we need to do is turn to Corbyn’s polar opposite on the parliamentary political compass: Margaret Thatcher.

Today, the Iron Lady can seem like a political and historical colossus that was always destined to be prime minister. But wind the clock back a little over 40 years and Thatcher, like Corbyn, had just thrown her hat into the race to be the next leader of the Conservative Party, following Ted Heath’s defeat in the two 1974 general elections.

We all know what happened next: Thatcher was elected Tory leader in 1975, and then Prime Minister in 1979, serving for an epoch-making 11 years. But what were her chances circa 1975? “This was a stunning transformation which no one would have predicted twelve months earlier: one of those totally unexpected events – which in retrospect appear predestined”, notes John Campbell in his biography of Thatcher about her successful bid to succeed Heath. Charles Moore, author of another biography of Thatcher, concurs: “Many regarded her candidacy as nothing more than a chance to prepare the ground for challenges by someone more serious, or merely for malcontents to let off steam.” The Guardian’s Anne Perkins simply notes Thatcher was seen as “unelectable” by her own party. Thatcher herself seems to have been one of the doubters, telling her Parliamentary Private Secretary “The party isn’t ready for a woman and the press would crucify me.”

She was right. According to Perkins, like Thatcher’s husband Denis, “every newspaper” thought “she was out of her mind”. The Sunday Mirror’s Woodrow Wyatt argued Thatcher would take the Tory Party in “an extremist, class-conscious, right wing direction” which would keep it out of office for a decade. She was “precisely the sort of candidate who ought to be able to stand, and lose, harmlessly”, argued The Economist. “Leading Heathites… believed that the Tory party must stay on the middle ground to stand a chance of winning elections”, according to Campbell.

If all this sounds strangely familiar four decades later, Moore provides another uncanny historical parallel: “As the campaign entered its closing phase, the ‘establishment’ which Mrs Thatcher had decided to take on pulled out all the stops.”

Of course, there are many significant differences between Thatcher’s and Corbyn’s bids to lead their respective parties and, if the Labour backbencher should win, their attempts to lead the country. However, my key - though obvious - point still stands: the unexpected can and sometimes does happen. And it can happen a lot quicker than anyone was expecting. There is, in short, no unchanging historical or political forces, or logical reason, why Corbyn couldn’t be prime minister.

In the 2009 Greek general elections Syriza received just 3.6 percent of the vote. Six years later they were in government. Spanish anti-austerity party Podemos formed in January 2014. 10 months later a poll found they had become the most popular party in Spain. UKIP won the 2014 European elections. In 2010 the SNP won six seats in the General Election. In May the SNP routed Labour, winning 56 of the 59 Scottish seats. In 2008 a black man became president of the United States of America.

As Tony Benn was fond of saying about grassroots social change: “First they ignore you, then they say you're mad, then dangerous, then there's a pause and then you can't find anyone who disagrees with you.” Benn’s dictum is right on the money when it comes to his friend’s leadership campaign. In June Polly Toynbee literally erased Corbyn from the race, arrogantly debating the prospects of “the three main contenders” before settling on the shadow Home Secretary Yvette Cooper as the most promising candidate. Earlier this month the Guardian published a comment piece from former Home Secretary Alan Johnson titled ‘Why Labour should end the madness and elect Yvette Cooper’. And, in a little covered interview, Conservative heavyweight Ken Clarke warned: “Don’t underestimate Jeremy Corbyn… It’s not certain he will lose an election… If you have another recession or if the Conservative government becomes very unpopular, he could win.”

Clarke is no doubt even more concerned by the recent Survation poll that found Corbyn is more popular with the electorate than the other candidates, particularly with UKIP supporters. The survey noted Corbyn was tied in first place with Andy Burnham when respondents were asked who they thought would be the most likely to win the next general election as Labour leader. In addition, a YouGov poll for the Evening Standard found Corbyn “has more support among the London public than his nearest rivals, Andy Burnham and Yvette Cooper, put together.” The fact that many of Corbyn’s political positions are supported by a majority of the general public can only increase his electability.

So, before politicians, journalists and, yes, Corbyn supporters, blindly repeat the establishment-framed and establishment-preserving argument that Jeremy is unelectable, they would do well to ponder the words of Nelson Mandela, a man who knew a thing or two about political change: “It always seems impossible until it’s done.”