“A liberal is a man too broad-minded to take his own side in a quarrel,” goes the famous adage, often attributed to the poet Robert Frost. If there was ever a time for liberals to take their own side, it is now. What was once hailed as the only remaining form of viable politics — the system of governance sure to bring about the fabled “end of history” — is under widespread attack. Ever since right-wing authoritarians the world over set about demolishing democratic protections, history has resumed with a vengeance. But liberals still have little to say for themselves: They often act as though their best selling point is that their competitors are even less appetizing than they are.