When President Obama took the stage here Wednesday to address a community—and a nation—traumatized by Saturday’s shooting 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 President Bill Clinton led after the bombing of a federal office building killed 168 people in Oklahoma City in 1995. But Mr. Obama’s appearance presented a deeper challenge, reflecting the tenor of his times. Unlike those tragedies-which, at least initially, united a mournful country and quieted partisan divisions—this one has, in the days since the killings, had the opposite effect, inflaming the divide. It was a political reality Mr. Obama seemed to recognize the moment he took the stage. He directly confronted the political debate that erupted after the rampage, asking people of all beliefs not to use the tragedy to turn on one another. He called for an end to partisan recriminations, and for a unity that has seemed increasingly elusive as each day has brought more harsh condemnations from the left and the right. It was one of the more powerful addresses that Mr. Obama has delivered as president, harnessing the emotion generated by the shock and loss from Saturday’s shootings to urge Americans "to remind ourselves of all the ways that our hopes and dreams are bound together.\ But Mr. Obama’s appearance presented a deeper challenge, reflecting the tenor of his times. Unlike those tragedies—which, at least initially, united a mournful country and quieted partisan divisions—this one has, in the days since the killings, had the opposite effect, inflaming the divide.