The attack comes as Portland has weathered rising tensions between groups with fiercely opposing views, including white nationalists, right-wing extremists, anarchists and so-called left-wing “antifascists,” known as the “antifa.” These groups have been colliding with increasing frequency across the country since the election last fall of President Trump — especially in cities regarded as liberal. In April, Berkeley, Calif., was the scene of riots as protests and counter-protests swirled around planned appearances of conservative commentator Ann Coulter and Milo Yiannopoulos, a former Breitbart News editor.