President Trump on Friday accused James B. Comey, the former F.B.I. director, of lying under oath to Congress in testimony that the president dismissed as a politically motivated proceeding. Mr. Trump also asserted that Mr. Comey’s comments on Thursday, in which the former F.B.I. director implied that the president fired him for pressing forward with the Russia investigation, had failed to prove any collusion between the Trump campaign and Moscow nor any obstruction of justice. “Yesterday showed no collusion, no obstruction,” Mr. Trump said in the White House Rose Garden, during a news conference with the visiting Romanian president. “That...