UK 12 September 2015 The epic challenges facing Jeremy Corbyn as Labour leader No leader has ever won office with a larger mandate, nor with so little support from their MPs. Jeremy Corbyn on stage after being announced as the new Labour leader at the QEII Centre in Westminster. Photograph: Getty Images. Sign UpGet the New Statesman's Morning Call email. Sign-up Jeremy Corbyn's landslide victory - the largest mandate ever won by a party leader - will at least come as no suprise to him. For a month, since the second YouGov poll putting him ahead, almost no one in Labour had doubted that he would triumph. Corbyn, who never contemplated winning when he made it on to the ballot, has had time to adjust to his epic ascent. But that does not lessen the scale of the challenge he faces. No leader in the party's 115-year history has ever been elected with so little support from MPs. Corbyn, the most left-wing figure to hold the post, won the public backing of just 14 (6 per cent) of his colleagues. As well as a traditional left-right split, Labour has suffered a severe schism between its MPs and its members. Corbyn's first task, as his acceptance speech underlined, will be to try and bridge the divide. He will need to deliver on his promise to assemble a broad-based shadow cabinet, rather than one solely comprised of the hard left. A large number of senior frontbenchers, including Yvette Cooper, Chris Leslie, Tristram Hunt, Liz Kendall, Michael Dugher, Mary Creagh and Emma Reynolds, will not serve under Corbyn. But others such as Owen Smith, Angela Eagle and Hilary Benn, from the soft left of the party, are prepared to join his team. By far the most important appointment Corbyn will make is that of shadow chancellor. John McDonnell, his greatest parliamentary ally and campaign manager, has long been the favourite for the post. But Corbyn has been warned by MPs that such a factional appointment would undermine his standing. Eagle and Smith are commonly cited as alternative candidates. But Corbyn may feel compelled to choose McDonnell out of personal loyalty. He is likely, however, to moderate some of his policy stances in order to persuade MPs to serve. He has already signalled that he will not advocate positions such as Nato withdrawal, anti-monarchism and opposition to EU membership as leader. Corbyn is likely to sideline the divisive issue of nuclear disarmament by holding a strategic defence review and may later offer a free vote on Trident renewal. Having rebelled against the whip 534 times since 1997, some colleagues believe that he will only loosely enforce discipline. Corbyn has pledged not to bring back mandatory reselection (abolished by Neil Kinnock), which would allow left-leaning constituency parties to purge opposing MPs. But in addition to a conference motion on the issue, the Tories' planned boundary changes will force many to face this test. Corbyn will come under immediate pressure to rein in his most radical supporters. A key figure amid the ructions will be Tom Watson, the party's new deputy leader, who warned against attempted purges in a speech last week. There is hope on the right and some fear on the left that Watson, who does not share Corbyn's politics, may later assist a move against him. After initially briefing that he would be ousted by Christmas, Corbyn's opponents have come to accept that the scale of his victory means he must be given a fair chance. They are also conscious of the need to avoid being blamed if he flounders early in his leadership. Next May's elections in England, Scotland, Wales and London are regarded as a defining test. Though some believe that the novelty of Corbyn will gift Labour an early poll bounce, most echo Dugher's prediction that the party will lose hundreds of councillors and suffer "dire consequences". John Mann told me that the Corbyn had "talked a big game" and that most MPs would judge him by results. "The Tories are rubbing their hands with glee but they also know Labour’s not going to tolerate any leader who performs disastrously in elections." The test for Corbyn's opponents will be whether they can unite around an alternative candidate - Dan Jarvis is frequently mentioned - should he enter troubled waters. The triumph of the left has convinced Labour's disparate factions - the Blairites, the Brownites, Blue Labour, the old right - that they need to form a unified moderate wing. Under the party's rulebook, rival candidates are required to attain the support of 20 per cent of MPs (46) in advance of the party’s annual conference. But in these circumstances, there would be nothing to stop Corbyn, or a left-wing successor (assuming they can secure 35 nominations), standing in the subsequent contest. Unlike in the 1980s, MPs can no longer play a decisive role under the new electoral system and the trade unions are a radicalising, rather than a moderating, force. For Corbyn, after 32 years as a backbencher, the job of leader of the opposition will be a profound culture shock. MPs regard prime minister's questions, relations with the media (who he ordered in his speech to "leave" his family "alone") and withstanding the Conservative assault as early tests of his credibility. Labour's left has won its greatest-ever victory but it is one that many believe it will never recover from. Corbyn's mission is to prove that they are as wrong about that as they were about his initial chances. › Labour chooses white man as leader George Eaton is senior online editor of the New Statesman. 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