Jeremy Corbyn started out as a 100-1 outsider, a token flag-bearer for the Labour left in a contest hardly anyone, himself included, thought he could win. By tapping into public anger over austerity and a yearning for authenticity, he rose from relative obscurity with a campaign that energised young party supporters, outmanoeuvred opponents and culminated in a resounding defeat for the centrist factions that have been in the ascendant in his party for more than two decades.
The 66-year-old veteran won by a landslide, his 59.5 per cent of the vote bigger even than the majority that swept Tony Blair to the leadership in 1994. It's a stunning achievement.
Now comes the hard part.
Corbyn may have the emphatic support of Labour members, giving him a mandate that will make it more difficult for would-be challengers to mount a heave before the 2020 election, but inside and outside the party he has enemies ranged against him. The Conservatives were quickly on the offensive, prime minister David Cameron tweeting: "The Labour Party is now a threat to our national security, our economic security and your family's security."
Triumphalist
Ministers have reportedly been told not to sound condescending or triumphalist. In one obvious sense the danger of dismissing Corbyn is real: the Labour leader’s unspun, down-to-earth approach, a key part of his appeal, is the antithesis of Cameron’s practised polish.
But while justice secretary Michael Gove remained on message, saying his party was not gleeful but "sombre" after Corbyn's victory, his words and body language betrayed the clear impression that the Conservatives see Labour's leftward shift as having enhanced their own chances of winning a third consecutive term in office.
Corbyn will also come under attack from the powerful Tory press. Yesterday's Sun carried a front-page mock-up of the new Labour leader in a bin ("In the Corbin") bearing the inscription: "Labour Party, 1900-2015". That's tame compared with the savaging that will follow, especially if Corbyn's Labour get an early bounce in the polls.
Serial rebel
But the new leader's biggest problems are internal. Less than 10 per cent of Labour's parliamentary party wanted him to win, and within hours of his victory a number of prominent MPs, including the defeated leadership candidates Yvette Cooper and Liz Kendall, announced they would not serve on his front bench. Corbyn peppered his post-victory comments with appeals for unity and has pledged to fill his shadow cabinet with a cross-section of opinion, but the serial rebel who broke records with the frequency of his own defiance of the party whip may struggle to enforce discipline from the top.
Already there are signs that the fissures that now run through the party hierarchy mean it cannot hope to speak with one voice. Tom Watson, the newly-elected deputy leader, yesterday declared loyalty to the party leader while admitting he disagrees with his views on Nato and the UK's nuclear weapons programme, which Corbyn wants to scrap.
The divisions in the parliamentary party feed into another of the colossal challenges Corbyn faces: managing his supporters’ expectations. His fans expect the radical rhetoric of the campaign to translate into a clear shift in policy direction.
Over 30 years in parliament, Corbyn has had the luxury of not having to compromise or strike deals with opponents. He has lent his name to the gamut of causes, and that record was one of the keys to his success. But like every political leader, he will eventually disappoint and disillusion some of the people who voted for him.
Nonetheless, Corbyn has a strong mandate to test the hypothesis behind his campaign: that Labour lost the last election by not being left-wing enough, and that a large swathe of the non-voting public can be mobilised behind his arguments on income inequality, austerity and foreign policy. But he will need good opinion poll showings and some signs of progress in next year’s elections for the Scottish parliament and the Welsh assembly to buy himself time and ensure his tenure doesn’t become bogged down in a protracted battle to hold the party together.