President Trump's campaign promise to eliminate the $20 trillion national debt was hyperbole, his budget director acknowledged in an interview aired Wednesday.

"It's fairly safe to assume that was hyperbole," Office of Management and Budget Director Mick Mulvaney said in an interview with CNBC. "I'm not going to be able to pay off $20 trillion worth of debt in four years. I'd be being dishonest with you if I said that I could."

That campaign pledge was always regarded as wildly implausible by budget experts.

Before joining the administration, Mulvaney was a South Carolina representative known as one of the most fiscally conservative members of Congress.

Yet, he said in Wednesday's interview that the public is not ready for the kinds of spending cuts that would be necessary to lower the debt. Instead, he suggested, the administration's priority is increasing economic growth, rather than lowering the debt.

In particular, Trump has opposed the kinds of changes to entitlement programs like Social Security and Medicare that Mulvaney said are necessary for debt reduction.

But Mulvaney himself still favors reforms to entitlement programs, and is trying to sway Trump on them. "We're working on it right now," he said.

Mulvaney said that he favors attaching reforms to an increase in the debt ceiling when that becomes necessary. He's still of the opinion, though, that the Treasury could prioritize payments on the debt if the ceiling is not raised, meaning that the consequences of Congress failing to raise the ceiling would not be as dire as commonly thought. Past Treasury secretaries of both parties have said that there is no alternative to raising the limit.