This has been a year when social democracy has had to confront existential demons. Sunday night’s surge by Podemos in Spain, to 20%, is just the latest challenge from the radical, populist and nationalist left that saw the SNP beat Labour in Scotland, and the far-left force its way into coalition in Portugal. Traditional socialist parties saw their territory captured by nationalist populism, too: Ukip in Britain, the Front National in France, the wipeout of the social-democratic left in Poland by its swing to the right in October.

Jeremy Corbyn’s seizure of the Labour leadership is an exception that demonstrates the rule: as his control solidifies, a whole generation of centrist politicians has begun to contemplate a breakway from one of the oldest socialist parties in the world, on the grounds that it is – as former Blair adviser Peter Hyman put it this week – “over”.

That the reversals were part of a long-term process is no consolation: long-term declines in politics tend to produce intermittent seismic events. The double election victory of Syriza this year, combined with its mobilisation of 61% of the population to defy austerity in a referendum qualifies as such an event. And whatever the outcome of the Spanish coalition negotiations, the seizure of Barcelona, Madrid and Valencia by radical-left coalitions in the local elections was seismic too.

So what’s driving the process? Though there are national peculiarities, the similarities are too obvious to ignore. First, the disintegration of class voting patterns noticed by sociologists from the late 1950s onwards. It’s often forgotten, amid the angst and panic of today, that the most fundamental challenge to social democracy was – long before deindustrialisation and neoliberalism – the fragmentation of class loyalty in politics. Next come the new demographics of modern societies. The industrial working class is small, even in succesful manufacturing countries like Germany; the salariat is large; the phenomenon of the young, networked, individualist only adds to social democracy’s existential problem, which is: whose values do we represent?

The salariat is liberal; the remains of the old, white manual working class can – if its concerns are repeatedly ignored and downplayed – become conservative. The networked individual thinks and acts globally; yet social-democratic machine politics has been essentially national and local for more than 100 years.

As identity politics gained traction, from the 1970s onwards, social democracy absorbed it successfully. But it has found it very difficult to absorb, respond or adapt to radical nationalism. Hence Labour’s collapse in Scotland, the marginalisation of traditional socialism in Catalonia and the Basque Country.

But the biggest problem of all is neoliberalism, and social democracy’s conversion to it. The neoliberal economic formula may have delivered growth and stability in the 1990s and early 2000s, but today it demands austerity, rising inequality, the erosion of welfare states to fund busted banking systems and the relentless reduction of labour’s bargaining power.

If you accept this, the question becomes: what would a non-neoliberal centrist socialism look like? But it’s a question few in the core socialist parties of Europe are prepared to ask. It challenges not only the leaders, but the footsoldiers – the apparatchiks who quit Labour HQ over Corbyn; the Blairite journalists mobilised across the newsrooms of Britain to do him down; the councillors who would rather he did not exist.

But 2015 has also begun to provide an answer. In Portugal, Spain and Greece the radical left parties have each had to make compromises – with power, with nationality and with more centrist forces. Syriza gained power in January by moderating its original programme and by recruiting and embedding numerousformer social democrats into its electoral offer. It was these politicians who urged moderation and compromise, eventually getting their way after Tsipras’s empty victory in the July referendum. But on another signal issue, Tsipras had already made the ultimate compromise. He had sidelined the issue of Nato, appointed the head of a small rightwing nationalist party as defence minister, and showed no embarassment himself at donning a military flack jacket to inspect troops.

Syriza ran the Greek capitalist state, though sometimes not competently. Its solution to an untrustworthy and politicised civil service was often to “squat” ministries, keeping physical distance from those parts of the machine Marxist theory tells you to keep your eye on closely: the military, the intelligence service, the diplomatic corps.In Barcelona, the Podemos-aligned En Comú movement, which took the city council in May, has been more radical – setting housing activists to run housing policy; instituting a crackdown on platforms like Uber and Airbnb. But Barcelona is not a state. In Portugal, these are still early days for the coalition of socialists, communists and radical leftists who squeezed through the constitutional hoops to gain power in November. But the price of the inclusion of the radical left in government was its prior commitment to honour Portugal’s debt repayments.Paradoxically, a mixture of realpolitik and the absence of monetary sovereignty has forced the radical left into a space that looks a lot like the answer to the question: a non-neoliberal social-democracy for the networked world.

If we consider what social democracy originally signified, it comes closer still: the workers’ parties that emerged in the 1890s chose the word sozialdemokrat, knowing it was a term of insult for Marxists. It meant relegating revolution, and the abolition of capitalism, to the status of a distant “maximum” goal, while being prepared to run capitalism in a more socially just way, according to a “minimum” programme of reforms.

Whatever Podemos, Syriza and Corbyn’s Momentum movement say they want, what they are actually proposing fits quite well with the maximum-minimum programme of the 1890s, except in one regard: the “maximum” goal has become woolly and centred around environmental targets rather than planned production and state ownership.

But this is not a steady-state solution. Politics in the developed world is challenging centrist structures from both right and left. With rightwing nationalism and social conservatism achieving, in many countries, about 25%, and the radical left pushing close to the same, there may not be room for more than one pro-global, pro-market centrist force in between the two.

It won’t happen suddenly, but the most likely outcome for European social democracy is the one being secretly contemplated on the Labour backbenches: a fusion with liberalised conservatism. So 2016 will be the year in which the true believers of centrist socialism will hear the message: “You can’t beat us, join us” from all sides.