Germany's political landscape was redrawn over the weekend with the creation of Die Linken ("The Left"), comprising eastern post-communists and a splinter group of disgruntled former Social Democrats (SPD).
The new party is headed by former communist Lothar Bisky and the former SPD leader and finance minister Oskar Lafontaine, who walked out of government in 1999 after feuding with then chancellor Gerhard Schröder.
After running a joint election campaign in 2005, the alliance made up of the two groups captured 8 per cent of the poll and is now the second-largest opposition party in the Bundestag.
In a fiery speech in Berlin, Mr Lafontaine said he was on a mission to tilt German politics to the left with his vision of "democratic socialism".
He painted The Left as the party of the German labour movement in the tradition of Karl Liebknecht and Rosa Luxemburg, a movement which he said had been betrayed by the welfare cuts of the government to which he once belonged. "We are the party of the social state. We need a new force, The Left, which says, 'yes, we want to restore the social state'," said Mr Lafontaine.
The movement was the new party of pacifism, he said - a knock at recent German military deployments - and the party of ecology, a play for disillusioned Green voters.
His 20-minute speech - short by German standards - was greeted with euphoric frenzy by the 800 delegates.
"I had the feeling that our comrades around the world were watching the moment when, for once, two left parties merged rather than split," said Anke Schneider, a former SPD member from Lower Saxony. "I looked around and saw tears in the eyes of my comrades around me."
By stealing the "social justice" clothes of the SPD, The Left hopes to vacuum up left-wing voters, not to mention left-wing SPD members frustrated with the party's reform strategy in the grand coalition.
Polling agencies give The Left a potential support basis of 24 per cent - rising to 44 per cent in eastern states. It already sits in the Berlin state government and, last May, entered its first western parliament in the city-state of Bremen.
With further gains likely at state elections next year, political analysts suggest that a "red-red" SPD-Left coalition is a question of when rather than if.
Until now, the SPD has ridiculed The Left and attacked Mr Lafontaine as a populist megalomaniac. They hope that the former finance minister will be a long-term liability for his new party: polls show that two-thirds of Germans don't want to see him in government again.
But even if it resists a red-red coalition, the SPD could be forced to revisit its left-wing roots ahead of the next general election to staunch the flow of voters to The Left.
In a sign of the changing times, SPD foreign minister Frank-Walter Steinmeier, considered by many the party's most promising candidate to run for chancellor, said at the weekend that his party "must take The Left very seriously".