“Despite this important progress, I continue to have concerns with NCUA's persistent failure to control expenses as your budget continues to balloon – nearly doubling in the last decade,” Mulvaney said, in the letter.

He also expressed his displeasure with the NCUA paying attorneys $1billion in legal fees in cases involving the sale of faulty mortgage-backed securities to five failed corporate credit unions. NCUA had hired attorneys on a contingency fee basis—a decision Mulvaney questioned.

Mulvaney also is co-sponsor of legislation that would require any overhead transfer of agency expenses to the insurance fund to be legitimate, substantiated, insurance-related costs.

NCUA has released detailed budget documents this year and held a public briefing on its spending.

Mulvaney also had a sharp disagreement with then-NCUA Chairman Debbie Matz during a committee hearing in July 2015.

While being questioned by Mulvaney, Matz twice said that she doesn't believe that credit unions represent their members

Mulvaney later referred to that exchange during a September 2015 speech at the NAFCU congressional caucus.

“It was stunning,” Mulvaney said. “To have oversight over an entire industry and think so poorly of the people in that industry was shocking.”

Matz left the NCUA board earlier this year.