No one knows what the future holds, but these pundits and politicians knew even less than the rest. If you took the sum of political knowledge available at the end of 2018 and projected the course of 2019, the resulting spaghetti models could go in millions of directions. Perhaps Robert Mueller’s findings would devastate President Donald Trump. Or maybe they would vindicate him. Perhaps the new House Democratic majority would steadfastly avoid impeachment. Perhaps Trump’s likeliest 2020 opponent would be one of several new faces heralded as the second coming of Barack Obama, like Beto O’Rourke or Kamala Harris. And...