Less than 48 hours after President Trump signed legislation ending the longest government shutdown in U.S. history, acting White House chief of staff Mick Mulvaney said the president would have no problem doing it again when funding is due to dry up again in three weeks.

"Yeah, actually I think he is," Mulvaney said on "Face the Nation," when asked if the president is willing to shut the government down again if he doesn't reach a deal with lawmakers over border spending. "He is willing to do whatever it takes to secure the border. He does take this very seriously. This is a serious humanitarian and security crisis."

Mulvaney said Mr. Trump "doesn't want to shut the government down" and prefers to address border security "the way that things are supposed to get fixed with our government, which is through legislation."

"We're still in the middle of these negotiations," he said. "The negotiations are far from over."

The bill Mr. Trump signed Friday night reopens the federal government for three weeks, a temporary reprieve from the stalemate that left hundreds of thousands of federal employees without pay. "We'll work with the Democrats and negotiate and if we can't do that, then we'll do, obviously we'll do the emergency because that's what it is. It's a national emergency," the president said last week while announcing the deal, referring to the possibility he could declare an emergency and free up funds for a wall.

On "Face the Nation," Mulvaney reiterated that working with Congress to get his border wall is the president's first choice over shutting down the government or declaring a national emergency. Mr. Trump "wants to do this the right way," he said.

"One of the reasons he agreed to open the government this week was to essentially take the Democrats at their word," Mulvaney said.

Mulvaney said Democratic members of Congress have indicated they want to work with the White House on the issue.

"There were many, many Democrats, both rank and file and some lower levels of leadership, [who] would come to us and say, 'Look, we happen to agree with you on border security,'" he said. "Some of them were even very public about it."

Mulvaney said Mr. Trump is so insistent on a barrier along the border because of the "humanitarian crisis" at the border.

Mulvaney also endorsed a tweet from Mr. Trump earlier Sunday morning claiming that the number of undocumented immigrants in the U.S. is much higher than 11 million.

"We know that it's going up because we know, for example, that 60,000 new illegals are coming across each month for the last three months. Again, a number that is not made up," Mulvaney said. "That is a real number. So we know the number has to be larger than 11 million. I've seen ranges as high, I think, of 30 or 40 million. I'm not exactly sure where the president got that number this morning. But I think what you see him trying to do is point out how silly this debate is."