The United Kingdom's Labour Party has a big problem, and that problem is named Jeremy Corbyn.

This Friday, the Labour Party will send out ballots for its leadership election. Corbyn, a member of Parliament from Islington North, is ahead by pretty large margins in some of the polls. And that's an absolute shock: Corbyn is best known for his radical left-wing views and comments, which include, for example, once referring to "our friends from Hezbollah." Labour's center-left establishment is terrified.

"If Jeremy Corbyn becomes leader it won’t be a defeat like 1983 or 2015 at the next election. It will mean rout, possibly annihilation," former Labour Prime Minister Tony Blair warns in a Guardian op-ed.

Yet Corbyn continues to do well in polls. "Corbynmania," as the British press has dubbed it, is sweeping the UK, with Corbyn's poll numbers remaining steady. He very well might still lose — but no one expected him to be doing this well. Here's why Corbyn is where he is, and what it tells us about British politics.

Why are people so freaked out about Corbyn?

To put it very simply: Corbyn's policy views are way out to the left of the Labour mainstream. Not all of his ideas are extreme, but enough of them are that the party chiefs fear they'd be unelectable if he led the party.

The BBC has an excellent rundown of Corbyn's actual policy platform. It includes, among other things, renationalizing Britain's railroad system and energy companies, abolishing tuition for British universities, and imposing rent controls to deal with Britain's affordable housing problem. He's even open to reopening the coal mines that used to be a big part of Britain's economy. It's essentially a throwback to the unreconstructed socialism — the real thing, way beyond Bernie Sanders — of the old-school British Labour Party, which used to be way more into the idea of the government controlling huge sectors of the economy.

Some of Corbyn's ideas are more appealing than others. Most importantly, he wants to end Britain's austerity spending cuts, which damaged the UK's recovery from the Great Recession. He also proposes something called "people's quantitative easing," in which the Bank of England would print money to invest in infrastructure projects. This won him praise from the Financial Times's Matthew Klein, who described it as a good way to get money into the hands of ordinary Brits and thus stimulate the economy.

Corbyn's positions on foreign policy are more extreme. He wants to withdraw from NATO, abolish the UK's nuclear arsenal, and has suggested that Blair could face a war crimes trial for his role in the Iraq War. His position on Ukraine echoes the Kremlin's: He's written that Russian expansionism "is not unprovoked" and that "the obsession with cold war politics that exercises the Nato and EU leaderships is fueling the crisis."

Notoriously, Corbyn once referred to members of Hamas and Hezbollah as "friends," and invited Hamas representatives to speak in Parliament. Here are the comments, from a 2009 speech he gave as a patron of the Palestine Solidarity Campaign:

It will be my pleasure and honor to host an event in Parliament where our friends from Hezbollah will be speaking. I’ve also invited our friends from Hamas to come and speak as well. … So far as I’m concerned, that is absolutely the right function of using Parliamentary facilities.

Corbyn has tried to play down the "friends" comments, arguing that he was just saying all parties to a conflict should be involved in peace negotiations. But they play into an all-too-well-founded belief that Corbyn is a real extremist on a number of important policy issues.

What happens if he actually wins?

No one's actually sure. But the Labour establishment is freaking out: They think a Corbyn victory would render the party unelectable, potentially forever. To understand why, you need to understand the internal ideological fights that have plagued the Labour Party for the past several decades.

Around the 1980s, Labour was repeatedly trounced by the Conservatives. Two Labour Party leaders — Tony Blair and Gordon Brown — blamed their party's left-wing platform for its losses, and became the leaders of a movement called New Labour. Think of it like a British equivalent of Bill Clinton and the Democratic Leadership Council: a force that pulled the party to the political center, particularly on economic issues, in the name of electability.

New Labour initially succeeded. It took over the party in 1994, when Blair was elected leader, and controlled the premiership from 1997 to 2010. But in 2010, the brand was retired after the Great Recession led to electoral defeat. However, New Labour's ideological influence is far from gone: Labour's mainstream and its leadership are still far more free market–oriented than they were in 1983, the year a landslide electoral defeat began the shift toward New Labour.

Corbyn's socialism, particularly his support for nationalizing chunks of the British economy, is a direct threat to Labour's current centrism. His critics accuse him of wanting to take the party back to the 1980s, or even the 1970s. A spokesperson for Yvette Cooper, a Labour MP and one of three leadership candidates competing against Corbyn, warned of "returning to the dismal days of the 1980s, with internal party warfare and almost two decades of [being in the] opposition."

Corbyn's fans, by contrast, see his candidacy as proof that today's Labour Party can finally renounce its centrist pretension and embrace its left-wing roots. "The Corbyn Surge, whatever it is, is a resounding comment on what has become of the worst of New Labour; an unflinching belief that Britain is a 'conservative country' and a 'centre' that must [be] chased not shaped," Neal Lawson writes in the New Statesman.

So the question of "what happens" if Corbyn wins really depends on your ideological read of the UK: Do you think Britain is ready for a hard-left Labour party? Or will moving Labour to the left just hand victory to the Conservatives?

Why are Labour voters turning to Corbyn?

The interesting thing about the Corbyn surge is that Labour, in a recent election, had tried moving to the left, and it got trounced. Its last candidate for prime minister, Ed Miliband, was sometimes called "Red Ed." Labour lost so badly that Miliband was forced to resign, and Conservatives won a surprising victory. So moving the party even further left seems like an odd political strategy.

That said, Labour also lost seats to the Scottish National Party, whose platform was distinctly to Labour's left. And in any case, the Corbyn surge is about something deeper than that: It's part of the backlash to austerity happening across Europe, which in the UK has combined with simmering anger at the New Labour years finally boiling over.

In more economically distressed countries, such as Greece and Spain, you've seen left-wing parties surge as part of a populist reaction to austerity cuts. The UK's economy has fared much less poorly. But Prime Minister David Cameron's spending cuts have been pretty painful for much of the population. They've also set back the British left's core political project, of expanding the welfare state to protect the vulnerable and promote equality. Cameron is with the Conservative Party, not Labour, but the backlash against his policies may have grown the support for a left-leaning Labour.

But Labour has been relatively timid in challenging the austerity cuts. Corbyn represents a real alternative: a full-throated rejection of austerity. That's an appealing vision to a lot of Labour voters frustrated with the direction their party has gone.

Indeed, Corbyn appears to have energized a newer wing of the Labour party. "People who joined the Labour party between 2010 and 2015 are more pro-Corbyn, [and] people who have signed up since 2015 are extremely pro-Corbyn," YouGov's Anthony Wells writes.

Can he win?

It's impossible to say at this point, but the polls seem to indicate he has a serious chance. The most recent poll, from YouGov, shows Corbyn with 53 percent support. The next most popular candidate, Andy Burnham, showed 21 percent support. Corbyn has support from the strongest labor unions, a critical constituency in the Labour leadership race. And UK betting markets have him as the odds-on favorite.

"Corbyn's odds have collapsed from 100/1 into 1/2 in the space of a few weeks," Matthew Shaddick, the odds compiler for the Ladbrokes betting agency, told City AM. "It now looks as if the Labour Party is going to deliver the biggest shock result in the history of political betting."

But it's far from over. Earlier in the election, polls other than YouGov's have shown Burnham leading. And the party's establishment, which pretty clearly opposes Corbyn, may find some way of blocking him.

The structure of the election also could hurt him. The vote uses something called the Alternative Vote, where voters are asked to rank the four candidates in order of preference. If no one gets an outright majority on the first ballot, then the person with the least votes is eliminated, and everyone who voted for that candidate gets their votes shifted to the next candidate. If there's still no majority, the second-to-last candidate gets eliminated, and his or her votes get distributed to the remaining two candidates, where someone has to have a majority.

Presumably, most of the people voting for Burnham, Cooper, or Liz Kendall — all more mainstream candidates — will rank Corbyn last. So even if Corbyn gets close to a majority on the first round, the Alternative Vote procedure could end up allowing "anyone but Corbyn" sentiment to carry the day.

But whatever happens after ballots go out on Friday, one thing is certain: Jeremy Corbyn represents the biggest ideological threat to the Labour establishment, and by extension the mainstream consensus in British politics, in the past 20 years. That's a really big deal.