EVERY DAY, President Donald Trump gives fresh reasons for Democrats to rue their election loss in 2024. The failures of Joe Biden and Kamala Harris mean that everything they professed to care about—the transatlantic alliance, the state’s capacity to improve society, the rule of law—is being unwound. Democrats spent years declaiming that a second Trump presidency would be catastrophic, an existential threat to American democracy. And yet more Americans preferred him to the alternative the Democratic Party set before them. Understanding how it all went so wrong would seem to be a priority.