The previously unreported remarks are the second known instance that Pruitt harshly criticized Trump during the 2016 presidential campaign.
"I think he's an empty vessel when it comes to things like the Constitution and rule of law," Pruitt said on the "Exploring Energy" radio show on February 11. "I'm very concerned that perhaps if he's in the White House, that there may be a very blunt instrument as the voice of the Constitution."
"This President, the one we have there now, has at least tried to nuance his unlawfulness," Pruitt continued, discussing then-President Barack Obama. "He at least sits back and says, 'How do we break the law and so where it's really tough to show that we have?' I'm not sure that Donald Trump would."
Earlier last week, Democratic Sen. Sheldon Whitehouse asked Pruitt about comments he made
on a different local Oklahoma radio show on February 4, 2016, where he said then-candidate Trump would be more abusive to the Constitution than Obama. Pruitt said he didn't recall those remarks nor did they reflect his current views on the President.
Asked about the newly discovered comments, a spokesperson for the EPA referred CNN to a statement Pruitt issued earlier in the week.
"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," Pruitt said. "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."
Earlier in the February 11, 2016, interview, Pruitt praised Trump as somebody who "has demonstrated and really responded to the frustration that people have" and for his willingness to take on China and other countries on international trade.
At the time of the interview, Pruitt was supporter former Florida Gov. Jeb Bush to be the Republican presidential nominee.
"We just need to make sure that we get somebody in there that respects the Constitution, respects the rule of law, that restores the proper balance between the states and federal government," Pruitt said. "I have great confidence Jeb Bush would do that. I have confidence that others would as well, but the time I spent with him, he's the most humble person I met and we need more humility in the White House and less hubris."