Photo credit to Stefan Rousseau/PA.

Within hours of the Labour Party’s unexpectedly severe loss in the UK general election, its leader Ed Miliband had already resigned and within days, Miliband and his family were Ibiza-bound, the first step in an awkward transition back to the backbenches. 

It’s been a gloomy downfall for Ed, and critics have good reason to criticize his performance between 2010 and the May elections. But what the Labour leadership contest aptly demonstrates is that Miliband nailed one thing in his five years — bridging the gap between Labour’s two factions, its union-heavy left and metropolitan, pro-business right. Miliband did such a good job maintaining Labour unity for the past five years, in fact, that no one realized just how divided Labour’s two tribes have become.

Those divisions are becoming all too clear in the emerging battle to succeed Miliband, and the surprise leader in the race is Jeremy Corbyn, a reluctant leadership contender first elected in 1983 to the British parliament, who’s now drawing the greatest support.

With the support of Unite, the most muscular of the labor unions that back the UK’s chief center-left party, Corbyn has managed to capture the imagination of a wide swath of the Labour electorate — disaffected nationalists, anti-austerity youths and old-school socialists like Corbyn (pictured above) himself, who’s bucked his party’s leadership 534 times in a 32-year parliamentary career. He bitterly opposed prime minister Tony Blair’s decision to join the US invasion of Iraq in 2003, he opposes the Trident nuclear deterrent and he’s an unabashedly ‘retro’ socialist. He’s not entirely committed to opposing a ‘Brexit’ from the European Union in the planned 2017 referendum. Corbyn wants to re-nationalize the UK’s train system and energy networks, and he wants to reaffirm public control over the health system.

By way of example, Corbyn makes no apologies for firmly opposing the government’s bill last week that trims social welfare benefits. Interim Labour leader Harriet Harman instructed the party’s MPs to abstain — Corbyn and 47 other Labour MPs joined the Scottish Nationalist Party (SNP) in voting against the legislation. It’s no exaggeration to say that a Corbyn-led Labour Party would embrace much of the 1970s and 1980s leftism that Blair worked so hard to expunge in the leadup to his 1997 landslide victory.

Corbyn’s rise is so stunning because he nearly missed the initial cut — he only barely achieved the 35 nominations from the party’s parliamentary caucus. He depended on support from legislators like Margaret Beckett, who believed that the far left deserved a voice in the campaign — if for no other reason than to show that the British far left is as weak in 2015 as in 2010 (when Diane Abbott placed last in the leadership contest) and 2007 (when John McDonnell failed to win enough nominations to advance against Gordon Brown, who won the leadership unopposed). Beckett, a former interim party leader and foreign secretary, now says she was a ‘moron’ to do so.

That’s because Corbyn, according to some polls, now holds a lead in the fight for Labour’s future. A July 17-21 YouGov/Times poll shows Corbyn leading with 43% of the vote — just 26% back shadow health secretary Andy Burnham (who held a series of ministerial profiles between 2007 and 2010), 20% back shadow home secretary Yvette Cooper (and the wife of former shadow chancellor Ed Balls and a former chief secretary to the treasury) and 11% back Liz Kendall, a shadow minister for care and older people first elected in 2010.

More alarming to the party establishment, when the choices are whittled down to the two leading candidates, Corbyn leads Burnham by a margin of 47% to 53%.

Ballots will not even be sent to Labour members until August 14, and the voting will continue through September 10 — there’s a lot of time left in the race, and it’s not clear Corbyn can sustain enthusiasm in the face of what will assuredly be a massive opposition to a Corbyn leadership.

But how, exactly, did a 66-year-old socialist become the pacesetter in Labour’s leadership contest?

Part of his rise is attributable to simple arithmetic.

In a four-candidate race, Corbyn monopolizes the support of Labour’s left wing. Burnham, Cooper and Kendall are all vying for the centrist wing of the party, divided though they may be on ‘Blairite’ or ‘Brownite’ lines. It stands to reason that Corbyn’s support could eclipse any of his contenders — at least on the first ballot. But under the rules of the leadership contest, there’s an automatic runoff until a candidate achieves a majority. As reformed in the Miliband era, voters will rank their candidates in an ‘alternative vote’ system. So the last-place candidate’s votes will be split among the remaining three candidates, on the basis of the voter’s second choice. So even a wide first-round Corbyn victory doesn’t necessarily guarantee he’ll survive subsequent rounds.

Part of his rise comes down to the fact that none of Burnham, Cooper or Kendall are particularly charismatic — none of them exude the easy touch of a Blair or even prime minister David Cameron. Kendall, the candidate unapologetically carrying the ‘Blairite’ torch, is derided on the hustings as a closet Tory, so fervently has she made the case that Labour has to embrace the center to return to power. But the more attractive Blairite candidate might have been another member of the 2010 class, Chuka Umunna, who dropped out of the contest just days after entering it — citing the media intrusion into his life and the life of his family.

Part of it is the sense that young voters, in particular, who came of age in the Blair-Brown era, see no fundamental difference between ‘New Labour’ and the Tories. The runaway success — even outside of Scotland — of the SNP’s Mhairi Black, the 20-year-old MP elected in May, is evidence enough of that.

Corbyn’s sudden popularity fits within a broader trend in favor of the hard left across much of the developed world. Greece now has the only far-left government in Europe, and the anti-austerity Podemos is running hard to lead the second in Spain’s upcoming general election. Thomas Mulcair and the New Democratic Party (NDP) are now leading polls as the most popular option ahead of Canada’s October elections, to the dismay of the center-left Liberal Party, now mired in third place under Justin Trudeau. In the United States, senator Bernie Sanders, a Vermont socialist, is waging a stronger battle than anyone imagined against former secretary of state Hillary Clinton for the Democratic presidential nomination.

Back in the United Kingdom, however, a Corbyn victory might just be enough to tear the party apart — as happened in 1981 when Roy Jenkins and David Owen defied Labour’s left-wing leader, Michael Foot, and bolted to form the Social Democratic Party (seven years later, it merged into the Liberal Democrats).

Even Blair chimed in, making it clear that Corbyn’s leadership would be devastating:

It would not take the country forwards, it would take it backwards. This is why when people say ‘My heart says I should really be with that politics,’  get a transplant.

Oddsmakers still give Burnham (pictured above) the edge. He’s young, but he has ample ministerial experience in government. Like Corbyn, he has union support, even as Labour elites would line up behind him. He has a pretty face, a working-class background in Liverpool and an accent that matches. Detractors say he’s a northern, third-rate version of Miliband; other critics argue he has no firm principles and will pander to the left and to the center.

In the immediate aftermath of the election defeat, the emerging consensus was that Miliband pulled too far left. Corbyn’s rise suggests the Labour grassroots feel differently.

If that feels somewhat schizophrenic, that’s because it is becoming the natural state of Labour these days. The party lost its stronghold in Scotland due to the SNP’s we-can-have-it-all approach to independence, autonomy and public spending. In northern England, concerns about immigration, economic opportunity and inequality caused traditional Labour supporters to consider the United Kingdom Independence Party (UKIP). In southern England, Miliband’s high-profile attacks on non-dom tax status and his call for a mansion tax seemed to reinforce that he was every bit the son of his Marxist intellectual father.

If the party chooses a Blarite leader like Kendall, it could win back the supporters of the 1990s and early 2000s — at the expense of Scotland and northern England.

If the party chooses Corbyn, it could stop the SNP and UKIP in their tracks — at the expense of horrifying the rest of England and the Labour elite.

If the party chooses Burnham or Cooper, it risks spreading itself too thin ideologically — thereby making it seem phony and inauthentic.

So no matter who emerges as Labour’s next leader on September 12, it will be vital to unite the varying factions. In a country where a Labour victory now depends on being different things to very different constituencies, the party’s tribal factions can no longer afford to snip at one another.