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”.