But the dispute over Iran has generated an unusual amount of friction, and Brazilian officials worried that failing to achieve progress in this weekend’s talks could cast Mr. da Silva as an amateur and scuttle his country’s pursuit of a permanent seat on the Security Council. Critics in Brazil have also questioned why Mr. da Silva has embraced Iran in recent months at the risk of alienating the United States. “There is a sense in Washington that a lot of this is a product of the tremendous confidence that Lula has in himself, that he believes he is a wizard that can perform miracles and accomplish what others have tried and failed to do,” said Michael Shifter, the president of the Inter-American Dialogue, a policy research group in Washington.