Three Republican former heads of the Environmental Protection Agency accuse the agency's current leadership of supporting the "undermining of science" and a potentially "catastrophic" approach to climate change.

In an exclusive interview with ABC News Live, before a rare joint appearance on Capitol Hill, former EPA administrators William Reilly, Lee Thomas and Christine Whitman warned that recent gains in cleaner air and water in the U.S. are beginning to "backslide."

"If we continue business as usual, it's catastrophic," said Reilly, who led the agency under President George H.W. Bush. "We're the number two emitter in the world after China."

Whitman, a former New Jersey governor who ran the EPA under President George W. Bush, raised concerns about "the disrespecting of science in the administration and the undermining of science and the importance of science."

Thomas said his old boss, President Ronald Reagan, would have wanted the agency to recommit to its central purpose.

"Change the direction, change the management and go back to the mission of the agency. That's what I think President Reagan would say, 'That's not how I want this agency to operate,'" Thomas said.

The trio spoke with ABC News before a House Energy and Commerce Subcommittee hearing Tuesday on the direction of the EPA, which was created in 1970 by Republican President Richard Nixon to protect the human health and the environment. Former Obama administration EPA chief Gina McCarthy also testified.

Asked to grade the performance of the current EPA leadership, led by Administrator Andrew Wheeler, the three Republicans agreed on "D" or "a little lower."

"You need an agency that is credible, has consistency in its rule making and is science-based," said Thomas, who lamented a steady exodus of career employees from the EPA because of disillusionment with its current direction.

When ABC News Live spoke with Wheeler earlier this year, he insisted that the agency remains focused on its core purpose but is balancing other economic priorities. He said he does not believe climate change is a "hoax," as President Donald Trump has insisted, but also does not think it's the "crisis" many environmentalists make it out to be.

"That's an unfair characterization," said Reilly of Wheeler's position, noting that the agency's own National Climate Assessment has painted a dire picture of the situation.

"The Green New Deal did something really wonderful," he said. "I am told by so many Republican members of Congress that they are never asked about climate when they go on the stump. And now, they're asked. That is marvelous. It's put it on the agenda."

Whitman agreed.

"For the first time in my memory, an environmental issue is going to be a major part of a political campaign for the presidency," she said. "I haven't seen that before, and it's not going to go away, and that's good."

WATCH the full ABC News Live interview here.

ABC News' Heidi Jensen contributed to this report.