WASHINGTON - As an independent candidate for the White House, Robert Kennedy Jr claims he would be the “best environment president in American history”, drawing on his past as a crusading lawyer who went after polluters in New York.

But dozens of Mr Kennedy’s former colleagues at the Natural Resources Defence Council are calling on him to withdraw from the race, in full-page advertisements sponsored by the group’s political arm that are expected to appear in newspapers in six swing states on April 21.

Separately, a dozen other national environmental organisations have issued an open letter calling Mr Kennedy a “dangerous conspiracy theorist and a science denier” who promotes “toxic beliefs” on vaccines and on climate change.

People involved in both efforts maintain that he cannot win the presidency but could siphon votes away from President Joe Biden and help elect former president Donald Trump, who has called climate change a hoax and promised to unravel environmental laws and policies.

“A vote for RFK Jr is a vote to destroy that progress and put Trump back in the White House,” says the newspaper ad that will run in Georgia, North Carolina, Nevada, Michigan, Wisconsin and Pennsylvania. Signatories include Mr John Hamilton Adams, who co-founded the Natural Resources Defence Council and hired Mr Kennedy in the 1980s, as well as past presidents and the group’s current president. They implore Mr Kennedy to “honour our planet, drop out”.

Mr Kennedy was a senior attorney for the Natural Resources Defence Council for about 28 years, stepping down in 2014.

In an interview on April 17, he shot back against the idea that he might bring Trump back to the White House.

“President Biden does not need my help to lose to Donald Trump,” he said. He avoided directly addressing the actions of Mr Adams and other former colleagues, saying only that he and his mentor “disagree with each other on politics”.

Instead, Mr Kennedy criticised Mr Biden as well as the environmental movement, which he said “is making a mistake to settle for crumbs that have been given to us by the Biden administration”.

Former colleagues in environmental circles were unvarnished in their assessments of Mr Kennedy. “The Bobby I knew is gone,” said Mr Dan Reicher, who worked with Mr Kennedy at NRDC. “His actions are a betrayal to our environment.”

The rebuke from Mr Kennedy’s professional colleagues comes after his brothers and sisters and other members of the Kennedy family endorsed Mr Biden on April 18. NYTIMES