A physical barrier would not stop undocumented immigrants from crossing the Mexican border and ranchers at the border say they don't need a fence, Mulvaney said in a 2015 interview uncovered by KFile.

As Trump's man in charge at the White House, however, Mulvaney would be tasked with leading the administration's push to build a wall along the US-Mexico border.

"Donald Trump says, build a wall. Deport all illegal immigrants. Rules are rules. You either play and stay or you cheat and you get deported. What challenges does this plan pose?" Patti Mercer asked in the August 25, 2015, interview on WRHI radio in South Carolina.

"A bunch," Mulvaney responded.

"The fence doesn't solve the problem. Is it necessary to have one, sure? Would it help? Sure. But to just say build the darn fence and have that be the end of an immigration discussion is absurd and almost childish for someone running for president to take that simplistic of [a] view," Mulvaney added, without making a distinction between a fence or wall.

KFile uncovered the comments as a government shutdown looms over funding for Trump's border wall. "Shutdown today if Democrats do not vote for Border Security!" Trump tweeted Friday.

A spokeswoman for Mulvaney, nor the White House returned a request for comment.

Mulvaney emphasized that the best way to protect the border is to enforce current law with more manpower and improved technology -- a policy the Trump White House is also pursuing.

"And by the way, the bottom line is the fence doesn't stop anybody who really wants to get across," Mulvaney said in the 2015 interview. "You go under, you go around, you go through it. And that's what the ranchers tell us, is that they don't need a fence. What they need is more manpower, and more technology, and more willingness to enforce the law as it exists today. There are parts of our border that are secure and parts of our border that are not. A lot of that comes down to whether or not we are just willing to enforce the law as it exists. So it's easy to tell people what they want to hear, 'build the darn fence, vote for me.'"

The instance is another example of Mulvaney attacking Trump during the presidential campaign. It comes as he is taking over as acting chief of staff, replacing Gen. John Kelly.

Last Friday, The Daily Beast reported that as a congressman, Mulvaney said Trump was "a terrible human being" during a November 2016 congressional debate. CNN's KFile reported on Monday that Mulvaney said in October 2016 Trump would be disqualified from office in an "ordinary universe." In both instances, Mulvaney said he was still supporting Trump over Democratic presidential candidate Hillary Clinton.

Earlier in the interview, Mulvaney said his views on immigration were shaped by talking to Texas ranchers.

"The fence is an easy thing to sell politically," Mulvaney said. "It's an easy thing for someone who doesn't follow the issue very closely to say, 'oh, well that'll just solve everything, build the fence.' When you go out and you talked to the Texas ranchers. Which I've done, [Rep.] Jeff Duncan has done, and ask them -- these are the guys dealing with it every single day."

"Because we have a fence there," the interview said. "And they're building tunnels."

"Yeah we have a fence, Mulvaney responded. "That's exactly the point."

Mulvaney added he believed Trump's emotional appeals and cavalier attitude might prompt him to try going around the Constitution.

"I wonder who is more interested in going around the Constitution in order to get things done. Barack Obama or Donald Trump," Mulvaney said.

"It's easy to tell people what they want to hear. It's easy to appeal to people's emotions," Mulvaney added. "I'm angry. I get it. I mean, believe me, I'm as angry as anybody on this. I do this for living. I have to deal with this administration every single day. It's extraordinarily frustrating. I feel that anger because I live with it and I talk to people about it."

"That's what I do when I come home, and that's people are angry," he continued. "And I get it because I'm as angry as they are, but just to appeal to somebody's emotion and say, vote for me because I'm as angry as you are, doesn't really solve the problem. It might make you feel better in terms of having voting with somebody who sympathizes with you, but it doesn't really solve the problem and that's what I'm interested in trying to do is, how do you solve the problems and winning elections is part of solving the problems."