Two Republican former Environmental Protection Agency chiefs are warning that a Trump presidency would not only  set the world back decades,  but could also be a direct threat to the environmental legacies of presidents Ronald Reagan, Richard Nixon, and George H.W. Bush. As a result, they have decided to back Hillary Clinton rather than the Republican nominee.William D. Ruckelshaus and William K. Reilly issued a statement on Tuesday announcing their endorsement of Clinton.Republicans have a long history of support for the environment dating back to Theodore Roosevelt. Donald Trump threatens to destroy that legacy of respect for the environment and protection of public health,  the statement read.Ruckelshaus was EPA administrator under both Reagan and Nixon. Reilly served under Bush. They both praised the environmental policies of the administrations they had worked under but said Trump doesn t have the slightest clue. Donald Trump has shown a profound ignorance of science and of the public health issues embodied in our environmental laws,  they said.  He hasn t a clue about Republicans  historic contributions to science-driven environmental policy. The former EPA heads also slammed Trump for his claim that climate change, which they called  the singular health and environmental threat to the world today,  is nothing more that a  hoax. It  flies in the face of overwhelming international science and the public conviction and commitment of almost 200 national governments that adopted the Paris Agreement on climate change in December 2015,  they said, pointing out that their  leadership was essential to that agreement. To back away now, as Trump wants to do, would set the world back decades years we could never recover. The young people in this country deserve far better than that as our legacy,  they addedThey wrote that Clinton, on the other hand, acknowledges climate change as the threat that it is and is concerned about protecting the environment. She is committed to reasonable, science-based policy to meet those challenges,  the men wrote. For us, there is simply no choice in this election,  they concluded.  We Republicans should be shocked, outraged even, at the prospect that all this progress, this legacy will be repudiated and rolled back by Donald Trump. This is a hugely consequential election; the stakes are that high. That is why as Republicans, we support Hillary Clinton for President. Featured image via Joe Raedle/Getty Images