They are struggling in Germany, reeling in France, on the defensive in Britain and clinging to power in Italy.
A decade after seizing control of governments across Europe, often with promises of a modern new "third way", the bloc's mainstream left-wing parties are in crisis, bickering over how to respond to the forces of globalisation and squeezed by rivals on either side of the political spectrum.
Evidence of the European left's decline can be found in Greece in the south, Poland in the east and The Netherlands in the west.
But its troubles are perhaps starkest in the continent's two biggest countries, Germany and France, where conservative Chancellor Angela Merkel and President Nicolas Sarkozy have coopted the left's ideas and their leading politicians.
Germany's centre-left Social Democrats (SPD), junior partners in Berlin's "grand coalition", have seen their support dwindle to multi-year lows of 25 percent in polls as voters abandon them in favour of Ms Merkel's more compassionate conservatives and a new far-left grouping called Die Linke.
Like France's Socialists, who are struggling to recover from this year's double defeat in presidential and parliamentary elections, the SPD has been plagued by an internal tug-of-war between moderates, who advocate a reform of the welfare state, and party leftists who defend it at all costs.
The divisions have diluted their profile with core blue-collar voters, who have fled to parties offering a clearer message on issues like globalisation, security and immigration.
"I think these two parties are confronted with the same problem," said Emmanuel Le Masson, a political scientist at Universite de la Mediterranee in Aix-en-Provence.
"Some members believe the mechanisms of the welfare state are impossible to maintain in the current economic context while others want to keep the social compromises achieved since the end of World War II."
That divide was on display at an SPD congress last weekend, where the party produced a mix of new proposals, including a plan to unwind their own labour market reforms.
The splits within Europe's leading centre-left parties are in part the legacy of leaders like Gerhard Schroeder in Germany and Tony Blair in Britain, who moved their parties to the centre, making them indistinguishable in the minds of many voters with their conservative rivals.
British Prime Minister Gordon Brown has felt this acutely since replacing Mr Blair this past summer.
He faces pressure from some in his Labour Party to shift to the left at a time when young conservative rival David Cameron is gaining ground with issues like immigration and tax policy.
Voter fatigue may be partly to blame for Mr Brown's recent woes, and to a lesser extent those of the SPD in Germany. Labour has ruled in Britain for the past decade and Mr Schroeder led Germany for seven years until Ms Merkel narrowly beat him in 2005.
"People are fed up," David Denver, a politics professor at Lancaster University, said of British voters. "The government has been in power too long. The conservatives are the opposition party and when people are fed up that's where they go."
The French Socialists can't blame their woes on voters' desire for change. Mr Sarkozy's resounding victory over Socialist rival Segolene Royal this spring came after fellow conservative Jacques Chirac's 12-year reign in the Elysee Palace.
Unlike Germany's SPD, the Socialists do not face a formidable challenge from their left flank.
But their internal squabbling, aggravated by a deep split on the 2005 referendum on the EU Constitution, has been more damaging than those of Europe's other centre-left parties.
Mr Sarkozy has seized on this, wooing top Socialist Bernard Kouchner into his own government as Foreign Minister, and sending another leading leftist, Dominique Strauss-Kahn, to Washington to run the International Monetary Fund.
Like Ms Merkel and Mr Cameron, he has seized the initiative from the left on popular issues such as the environment.
Another big problem for the centre-left across Europe has been a dearth of new charismatic leaders.
Mr Brown lacks Mr Blair's rhetorical flair and common touch, while SPD leader Kurt Beck is seen as provincial and colourless next to his predecessor Mr Schroeder.
France's Socialists are still fishing for someone to lead them out of their misery, while Italian Prime Minister Romano Prodi is fending off attacks by his more flamboyant predecessor, conservative Silvio Berlusconi, who leads him in the polls.
Only in Spain, where Socialist Prime Minister Jose Luis Rodriguez Zapatero holds a solid lead over the conservatives ahead of March elections, is the left in good shape.
Josef Joffe, publisher-editor of German weekly Die Zeit and a fellow at Stanford University, believes it could take years for big leftist parties to recover, particularly the SPD which is operating in an increasingly crowded political field.
"Too many of the classic Social Democratic issues have been captured by the far-left, centre-right and the Greens," he said.