Angela Merkel's main challenger in next year's German election accused her on Thursday of allowing her coalition partners to abuse and bully European partners in a way previous leaders, including her mentor Helmut Kohl, would never have stood for.

In a combative speech to the Bundestag lower house of parliament on Thursday, Social Democrat (SPD) Peer Steinbrueck also accused Merkel of failing to sell the benefits of euro zone membership to German voters during the debt crisis.

"It was a grave mistake that you allowed your coalition to launch a bullying campaign against Greece's membership in the euro zone. You didn't intervene, you didn't speak out for Europe and you vacillated,» he said, thumping his fist on the podium.

"Neither (former conservative chancellor) Helmut Kohl nor any of your predecessors would have allowed a European neighbor to be abused for domestic political purposes like that."

In the summer, several lawmakers in Merkel's center-right coalition openly talked about a possible exit of Greece from the euro zone, with some using colorful mountaineering metaphors to illustrate why Germany should cut the rope to avoid being pulled into the abyss by its weaker partner.

Merkel, who had addressed the Bundestag before Steinbrueck, shortly before the start of an EU summit, stared glumly ahead and did not respond to the feisty attack from the man who will stand against hear in next year's election.

Steinbrueck also said Merkel had to come clean and tell Germans they would have to pay more for Greece.

"Looking at Greece, Germany will have to take on more obligations together with European countries - say it, finally,» said Steinbrueck, to cheers from the SPD benches. [Reuters]