David Remnick: “The emergency that the Trump Presidency represents leaves the Democratic Party’s cast of candidates with a singular responsibility—to win the election—and two colossal reclamation projects. The first involves the environment. Presidential debates in past elections have largely ignored the costs of climate change. But public opinion on the topic is moving, and there is cause for at least some political optimism in the fact that many Democrats have gotten behind the idea of a New Deal-scale effort to address the issue.