President Trump derided the Green New Deal as a “high school term paper that got a low mark.” Congressional Republicans mocked it as “zany.” Even Nancy Pelosi, the Democratic House speaker, called the proposal a “green dream,” and some of the party’s 2020 candidates are starting to describe it as merely aspirational. Yet, despite that disdain, the goals of the far-reaching plan to tackle climate change and economic inequality are within the realm of technological possibility, several energy experts and economists said in recent interviews.