President Donald Trump departs the White House on March 13, 2018, heading to California to view prototypes of a proposed border wall. (Photo by Mark Wilson/Getty Images)

(CNSNews.com) - President Donald Trump is signaling that he won't give up on his long-promised wall, which a Republican-controlled Congress failed to deliver -- and a Democrat-controlled House now vows to block.

On Sunday, Trump tweeted: "After all that I have done for the Military, our great Veterans, Judges (99), Justices (2), Tax & Regulation Cuts, the Economy, Energy, Trade & MUCH MORE, does anybody really think I won’t build the WALL? Done more in first two years than any President! MAKE AMERICA GREAT AGAIN!"

President Trump has indicated that he might declare a national emergency if Congress doesn't give him money for his wall in the next three weeks.

White House Chief of Staff Mick Mulvaney, appearing on CBS’s "Face the Nation," indicated that Trump is serious about it:

"Is the president really prepared to shut down the government again in three weeks?" host Margaret Brennan asked him.

"Yes, I think he actually is," Mulvaney said:

Keep in mind, he's willing to do whatever it takes to secure the border. He does take this very seriously. This is a serious humanitarian and security crisis. And as president of the United States, he takes the security of the nation as his highest priority. He doesn't want to shut the government down. Let's make that very clear. He doesn't want to declare a national emergency. What he wants to do is fix this the way that things are supposed to get fixed with our government, which is through legislation. And I think that has been our perspective from the very beginning, along with the principle that shutdowns are not legitimate negotiating tactics when there's a public policy disagreement between two branches of government.

But some members of Trump’s Republican Party balk at the idea of a national emergency declaration:

"I don't think it's a good idea. I think it will be a terrible idea," Sen. Marco Rubio (R-Fla.) told NBC's "Meet the Press" on Sunday. "I hope he doesn't do it."

Rubio said he's not sure if the president really would declare a national emergency to bypass congressional appropriators.

"I know it's an option they've looked at. But now you are at the mercy of a district court somewhere, and ultimately an appellate court. So it really may not even withstand if you look at some of the other rulings we've seen.

“The other is the precedent that it sets. And it's just not a good precedent to set in terms of action. It doesn't mean that I don't want border security, I do, I just think that's the wrong way to achieve it. It doesn't provide certainty, and you could very well wind up in sort of a theatric victory at the front and then not getting it done. I think the best way to do it is to have a law passed that funds border security so we know it's going to happen.”

Sen. Susan Collins (R-Maine) told CBS's "Face the Nation" that she hopes the next three weeks will produce an agreement that ends the threat of another government shutdown.

"The best agreement that we can get is an agreement on border security, but an agreement to fund federal government through the end of the fiscal year, which is September 30 -- no more short-term stopgap funding measures. And we cannot have the threat of a government shutdown hanging over our people and our economy," Collins said.

Asked if President Trump will end up with wall funding, Collins talked about "physical barriers."

"I think what will happen is that the efforts to continue to build physical barriers, which have gone on in the last two administrations, will continue, but not to the degree that the president has requested.

“What we should do is ask the experts, the nonpartisan experts at Customs and Border Patrol, what are their biggest problems? It's going to be a combination of physical barriers, technology, more Border Patrol agents, more immigration judges, more sensors. It's got to be an all-of-the-above approach.”