Mick Mulvaney speaks in the Brady press briefing room at the White House in Washington Image Credit: AP

One of Mick Mulvaney's first tasks as acting chief of staff to Donald Trump will perhaps be explaining why he previously publicly called his boss "a terrible human being".

Video has emerged of Mulvaney, previously a Republican congressman, admitting his disdain for Trump shortly before the presidential election in November 2016.

"Yes, I'm supporting Donald Trump; I'm doing so as enthusiastically as I can do. In fact, I think he's a terrible human being," Mulvaney said. Referencing Trump's opponent Hillary Clinton, Mulvaney added: "But the choice on the other side is just as bad."

The videoed remarks, published by the Daily Beast, occurred during a debate between Mulvaney and the Democrat Fran Person in York, South Carolina.

Following Trump's unexpected election win, Mulvaney became a key part of the administration, at one point being both the White House budget director and head of the Consumer Protection Bureau.

A staunch conservative, Mulvaney outlined deep federal spending cuts that Congress balked at, as well as a weakening of consumer protection rules.

He now becomes, temporarily at least, the latest chief of staff for Trump. John Kelly , a retired Marine general, is set to depart the role at the end of the year. Kelly, in turn, replaced Reince Priebus, Trump's first chief of staff.

Trump's hunt for a chief of staff has not gone smoothly and has been defined more by people declining the job than queuing up to fill it. His first choice was Nick Ayers, Vice-President Mike Pence's chief of staff, who bowed out after being unable to agree on how long he would serve in the post. On Friday, Chris Christie, the former governor of New Jersey, also ruled himself out of contention.