Several former colleagues of Robert F. Kennedy Jr. at the Natural Resources Defense Council (NRDC) and other national environmental organizations are calling on him to withdraw from the presidential race. Full-page advertisements, sponsored by the NRDC’s political wing, are schedule to appear in newspapers in six swing states on Sunday. “Earth to RFK Jr. … Drop Out,” says the ad. A dozen national environmental organizations have issued an open letter denouncing Kennedy as a “dangerous conspiracy theorist and a science denier,” promoting “toxic beliefs” regarding vaccines and climate change. …