These are troubling times for Europe’s social democrats. Centre-left parties face fresh threats next Sunday, when Italians will vote in their first general election for five years and Germany will learn whether its centre-left SDP will approve a new coalition with Angela Merkel’s centre-right CDU.

The two ballots are the latest tests for a mainstream centre-left which, outflanked on left and right, appears in full retreat – all but wiped out in France and the Netherlands, humiliated in Germany, struggling even in its Scandinavian heartland.

In Italy, an untested mix of first-past-the-post and proportional representation is now likely to produce a hung parliament, leading to months of horsetrading before an unstable right-left coalition is formed, possibly including the far-right Northern League (Lega Nord).

Final surveys suggest two other scenarios. Silvio Berlusconi’s centre-right Forza Italia and its anti-immigration allies could secure an outright majority. Or the anti-establishment mavericks of the Five Star Movement – set to be the largest party, with 26-28% of the vote – could perform well enough to try to form a government, probably as a minority administration.

None of the three outcomes look good for the ruling centre-left Democratic party (PD) headed by Matteo Renzi, who was hailed barely four years ago as the hope of Europe’s centre-left after he became Italy’s youngest prime minister in 2014. The PD is on course for barely 21% of the vote.

That would be one point more, however, than Germany’s Social Democrats (SPD) managed in disastrous September elections that saw the centre-left party record its lowest score since 1949, and the nationalist, anti-immigration Alternative für Deutschland (AfD) enter the Bundestag for the first time.

Following Merkel’s failure to negotiate a three-party government, the chancellor turned back to her previous coalition partner. The shell-shocked SPD, initially favouring a spell in opposition after losing more than half its voters in the past 15 years, reluctantly agreed. But the party’s 460,000 members must now approve a deal committing to another grand coalition that many feel risks condemning the party to the political wilderness and further advancing the far right.

They may be right. Support for the centre-left party sank throughout the coalition negotiations and last week the AfD edged ahead of the SPD for the first time, registering 16% in a usually reliable poll.

In Germany and elsewhere, many now believe that while rejection by the SPD’s rank and file of another “GroKo” (grosse Koalition) would certainly shake the country – not to mention the EU – in the short term, it may be the best long-term choice.

Rightwing populism cannot be stopped, they argue, without a regenerated and reinvigorated centre-left, reconnected to its base – which is unlikely to happen if it’s in an unpopular coalition with a centre-right party increasingly chasing the support of nationalists.

But Italy’s PD and Germany’s SPD are just the latest centre-left victims of the spectacular recent fragmentation of Europe’s political landscape, first exemplified by the shocking collapse of Greece’s Pasok social democrats from 43.9% of the vote in 2009 to just 6.3% in 2015.

In France, the Socialists under François Hollande held the Elysée palace, parliament, senate and a majority of regions in 2012, but finished fifth in last year’s presidential race and later crashed from 280 MPs to 30, when it won just 7.4% of the vote.

The Dutch Labour party, the PvdA, also suffered a crushing defeat, punished for backing the liberal policies of the outgoing centre-right-led coalition of which it was a part. It saw its party collapse from 38 MPs to nine after winning just 5.7% of the vote in last year’s election.

The reasons for the decline are many. The pro-market “third way” policies of leaders such as Tony Blair and Gerhard Schröder worked fine in the turn-of-the-century boom, but offer little to today’s vulnerable voters. The 2008 financial crash and its fallout – high unemployment, low living standards, public spending cuts – have combined with longer-term trends (globalisation, automation, immigration) to erode traditional centre-left support.

Populist far-right parties have also openly addressed precisely those concerns, attracting the support of many historically centre-left voters, while at the other end of the spectrum, a new anti-capitalist, anti-globalisation, anti-establishment far left has proved just as big a threat.

Across a continent, the moderate left that played such a key part in rebuilding postwar democracy is, incontestably, foundering. Unless it can once more offer voters credible solutions to their very real problems, its decline could be terminal.