Environmentalists who once worked with Robert F. Kennedy Jr. are calling on him to drop his presidential bid, The New York Times was first to report Friday, while a number of green organizations are also denouncing the independent candidate for his anti-science beliefs.

The push for Kennedy to exit the 2024 race comes from his ex-colleagues at the Natural Resources Defense Council, whose political arm is planning to run full-page newspaper ads in six swing states next week. Kennedy had served as a senior attorney at the climate advocacy group for around 28 years.

The ads describe his campaign as benefiting presumptive GOP nominee Donald Trump , the single worst environmental president our country has ever had.

We have spent our careers fighting to protect the planet and its people, the ads state. As current and former leadership and board members of the NRDC Action Fund, as well as former colleagues of Robert F. Kennedy Jr., we have one message for him: Honor our planet, drop out.

They further state that Kennedy has been spinning anti-vaccination conspiracy theories, denying science, and putting lives at risk.

The RFK, Jr. I knew as a colleague is not the same person I see today, Liz Barratt-Brown, one of his former colleagues, said in a statement.

We feel betrayed by him and what he is espousing. He is using his environmental credentials to sell a dark and conspiratorial brand of politics. Its heartbreaking. If he loves the planet the way he did as my former colleague, he cannot open the door to another Trump Administration.

Separately, Kennedy faced criticism Friday in an open letter from a handful of environmental groups, including the NRDC Action Fund. The 12 organizations rejected Kennedys claims of being an environmental candidate and sought to warn the American public about his beliefs, pointing to his previous anti-science statements.

Though he was once a leading environmental attorney, Kennedy is now peddling the term climate change orthodoxy and making empty promises to clean up our environment with superficial proposals, the letter says.

Robert F. Kennedy, Jr. is not an environmentalist, it argues, suggesting that his agenda would be a disaster for our communities and the planet.

The letter calls him a dangerous conspiracy theorist and science denier, citing reports on false claims hes made about vaccines causing autism in children and statements minimizing the riot at the U.S. Capitol on Jan. 6, 2021, among others.

The letter says that Kennedys rejection of science makes him no different than Donald Trump.

Kennedy is playing a dangerous spoiler role that could benefit Trump, Manish Bapna, the president and CEO of the NRDC Action Fund, said in a statement, adding that President Joe Bidens efforts and policies around climate change make him the real environmental candidate.

Meanwhile Kennedys campaign staffers have made clear they are trying to syphon votes away from the President. Polling shows he has no path to the White House, so if Kennedy is serious about protecting the planet, the time to drop out is now, Bapna said.

Kennedys campaign did not immediately respond to HuffPosts request for comment.

Trumps camp initially thought that Kennedys candidacy could benefit the former president. But now, both the Republicans campaign and Bidens worry that Kennedy could lose them votes.

A Kennedy presidency appears to be a long shot, with a Wall Street Journal poll last month finding that he averaged 11% support in seven states that will likely determine the 2024 election.

Meanwhile, more than a dozen members of the Kennedy family including the independent candidates sister endorsed Bidens campaign Thursday.

President Biden has been a champion for all the rights and freedoms that my father and uncle stood for, said Kerry Kennedy, the daughter of former U.S. Attorney General Robert F. Kennedy and niece of former President John F. Kennedy.

Thats why nearly every single grandchild of Joe and Rose Kennedy supports Joe Biden, she added. Thats right the Kennedy family endorses Joe Biden for president.

In a post on social media, Robert F. Kennedy Jr. acknowledged the news of his familys support for Biden.

I hear some of my family will be endorsing President Biden today, he wrote . I am pleased they are politically active its a family tradition. We are divided in our opinions but united in our love for each other.