The challenge is becoming increasingly clear for House Democrats prosecuting President Donald Trump’s impeachment case as the Senate convenes for a second day of arguments in the landmark trial. No matter how overwhelming the evidence confronting Trump, it becomes less compelling when presented again and again, day after day, as Democrats try to convince not just fidgety senators but an American public deeply divided over the Republican president in an election year. The team led by Rep. Adam Schiff, the chairman of House Intelligence Committee, constructed a gripping account of Trump’s political pressure on Ukraine and attempt to cover up...