"Donald Trump has said many, many times ... 'I'll do this, I'll do that.' And those things that he's mentioned cannot be done. I think executive orders with Donald Trump would be a very blunt instrument with respect to the Constitution," Pruitt said on the program, broadcast on KFAQ Talk Radio 1170 in Tulsa, Oklahoma.

"I believe that Donald Trump in the White House will be more abusive to the Constitution than Barack Obama , and that's saying a lot," Pruitt, a Republican, told radio host Pat Campbell.

Environmental Protection Agency administrator Scott Pruitt said in a 2016 interview that if Donald Trump were elected president, Pruitt believed he would be "abusive to the Constitution" and would insist on using his executive powers in ways that were "truly unconstitutional."

"This president has basically taken an approach that says that 'if Congress doesn't act or they don't act the way that I want them to, then I can act in their place,'" Pruitt said on the program, which was broadcast Feb. 4, 2016. Pruitt was attorney general of Oklahoma at the time.

Pruitt also said that if "Donald Trump is the nominee and eventually the president, he would take, I think, unapologetic steps, to use executive power to confront Congress in a way that is truly unconstitutional."

Asked about the comments during a Senate hearing on Tuesday, Pruitt said he could not remember making them. An hour later, Pruitt released a statement praising the president for "advancing the rule of law."

Pruitt's statement read: "After meeting him, and now having the honor of working for him, it is abundantly clear that President Trump is the most consequential leader of our time. No one has done more to advance the rule of law than President Trump. The President has liberated our country from the political class and given America back to the people."

This is not how Pruitt felt about Trump in February of 2016, however. At the time, Pruitt was supporting the Republican presidential campaign of Sen. Ted Cruz of Texas. When it became clear that Trump would be the party's nominee, Pruitt shifted his support to the New York real estate developer, as did many other prominent Republicans.

What makes the unearthed Pruitt comments so problematic, however, is that within the Trump universe, they represent precisely the kind of betrayal for which Trump rarely forgives people.

Following Trump's upset victory in 2016, scores of career Republican national security professionals feared they were blackballed from getting jobs in the new administration because they had added their names to a letter critical of Trump during the campaign.

It's unclear how Trump will respond to Pruitt's comments, now almost two years old. The Oklahoma lawyer, who made his name suing the EPA, has shown himself willing and able to carry out Trump's policy agenda, which emphasizes fossil fuel production and deregulation.

A White House spokesman did not immediately reply to a request for comment from CNBC.