In the awkward aftermath of the Green New Deal’s rollout, perhaps the most appropriate question for its supporters, especially the Democratic presidential field, is one often posed by tennis bad boy John McEnroe: “You cannot be serious!” But, apparently, when New York Rep. Alexandria Ocasio-Cortez and Massachusetts Sen. Edward Markey introduced their proposal in February, they were deadly serious, and breathless progressives couldn’t wait to hop aboard the climate change express. First in line, the Democratic presidential candidates in the Senate who were eager to offer up their enthusiastic support. There was just one snag. The Green New Deal, in...