Rivals Joe Biden and Donald Trump urged Americans to show unity on Sunday after an assassination attempt on the Republican that the FBI said was carried out by a shooter with a legally bought semi-automatic rifle. The attack put the divided nation on edge in the run-up to the presidential election and was being investigated as a potential act of domestic terrorism, as well as a massive security failure. President Biden said he would address the nation from the Oval Office later in the day, a step only taken at times of grave crisis, after his 78-year-old predecessor was hit in the ear at a cam…