The epithet “climate denier,” intended to invoke Holocaust denial, has always been tasteless and inapt. Climate change is not like the Holocaust, nor is questioning the accuracy and predictive power of a scientific model like questioning the historical fact of a genocide that murdered 6 million Jews. But climate activists delighted in defining their opposition this way, with help from prominent figures such as Barack Obama, who in 2014 used Twitter to condemn “climate change deniers” and promote a website, run by Organizing for Action (formerly Obama for America), that featured large black-and-white pictures of then–House speaker John Boehner and Senator Marco Rubio atop a green “Climate Change Deniers” banner. “On climate,” asked the site’s headline, “whose side are you on?”