Define 'succesfull'. If you mean electorally succesfull, than yes, at that point in history New Labour was probably the left's biggest succes, altough Canada's Liberals had been in office longer and with bigger majority's and had yet another minority government to come. Likewise if you mean influential. But in terms of realizations, I wouldn't know. Schröder did realize some pretty important things, and there are others out there who might have a claim to that throne.
Personally, I feel the purple governments in the Bene(lux) also were rather succesfull, even if the 2002-2004 period was the point when they started to wane. (The Netherlands: 2002 election and Fortuyn, Belgium: 2004 regional elections, just a year after the epic 2003 federal elections
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