When President Obama took the stage here Wednesday to address a community — and a nation — traumatized by Saturday’s shootings rampage in Tucson,Arizona, it invited comparisons to President George W.Bush’s speech to the nation after the attacks of Sept. 11, 2001, and the memorial service PresidentBillClinton led after the bombing of a federal office building killed 168 people in OklahomaCity in 1995.