The environmental chiefs from two past Republican administrations endorsed Democratic presidential nominee Hillary Clinton on Tuesday, saying GOP nominee Donald Trump is too ignorant for them to support.

"Donald Trump has shown a profound ignorance of science and of the public health issues embodied in our environmental laws," the former heads of the Environmental Protection Agency said in a letter sent to Clinton. "He hasn't a clue about Republicans' historic contributions to science-driven environmental policy: the incontestable environmental improvements in the restored lakes and rivers, the acid rain controls, the reduction of key air pollutants by 70, 80 and 90 percent, and the sharp decline in pollution from automobiles even as their numbers more than doubled."

The former EPA chiefs include William Ruckelshaus, who served under Presidents Richard Nixon and Ronald Reagan, and William Reilly, who headed the agency under President George H.W. Bush.

They also said that Trump's view on climate change "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. Our 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."

Clinton, they said, "is committed to reasonable, science-based policy to meet those challenges."

Trump has said he would rescind last year's Paris climate change pact among the U.S and nearly 200 other countries and cancel EPA climate and clean energy regulations that have become the centerpiece of President Obama's environmental agenda.