With the pillars of the Brandenburg Gate looming in the background, tens of thousands of adoring Berliners turn out to greet Senator Barack Obama with wild cheering. It may have seemed like the perfect campaign stop on the candidate’s highly anticipated European tour, an ideal way to burnish his foreign policy credentials.

Instead, the plan — widely dissected in the German news media but never confirmed by the Obama campaign — has exposed fissures in the German government, with the conservative chancellor, Angela Merkel, strongly criticizing the proposal and the Social-Democratic vice chancellor and foreign minister, Frank-Walter Steinmeier, speaking out in favor of it. The ensuing uproar here has underscored the hazards for Mr. Obama in taking his presidential campaign overseas.