Germany's Social Democrats will launch their election campaign Sunday. By steering to the left and attacking Angela Merkel, the party wants to score big with voters. Its strategy will also force the conservatives to inch leftward. In the midst of a global downturn in which stimulus is the buzzword of the day, economic prudence could take a backseat.

A woman holds up playing cards with caricatures of German Chancellor Angela Merkel and her opponent in this year's election, Foreign Minister Frank-Walther Steinmeier.

On Sunday, Germany's center-left Social Democratic Party plans to announce its manifesto for its campaign in the run-up to the country's Sept. 27 national election. Aides of the party's chancellor candidate, Frank-Walter...