Transcript for Trump declares national emergency in the Rose Garden

We turn to president trump who went on live TV from the rose garden and declared a national emergency citing an invasion on the border. He's trying to go around congress to get billions more for his wall. Tonight Democrats and the growing list of Republicans who say this move could create a slippery slope for future presidents. Our Jonathan Karl asking the president about the Republicans who don't want this. Reporter: In the rose garden, the president did what he had long threatened to do, declaring a nationalency to get billions of dollars congress wouldn't give him to build his border wall. We're talking about an invasion of our country. Reporter: Congress only approved $1.4 billion for the wall, but the white house says the president can get an additional six and a half billion using executive and emergency powers to shift money from other projects. Most of it would come from the Pentagon, money that is intended for counter narcotics programs and emergency military construction. The president insists military leaders are onboard. They think this is far more important than what they were going to use it for. I said what were you going to use it for and I won't go into details but it didn't sound too important to me. Reporter: Speaker of the house Nancy Pelosi called the emergency declaration a power grab, and at least ten Republican senators criticized the move. Marco Rubio saying, "No crisis justifies violating the constitution." Reporter: Mr. President, what do you say to those including some of your Republican allies who say that you are violating the constitution with this move and setting a bad precedent that will be abused by possibly democratic presidents in the future? Marco Rubio has said that. Not too many people have said that, but the courts will determine that. Reporter: The president made it clear he fully expects to be sued. We will have a national D we will then be sued and they will sue us in the ninth circuit, even though it shouldn't be there, and we will possibly get a bad ruling and then we'll get another bad ruling and then we'll end up in the supreme court and hopefully we'll get a fair shake. Reporter: In court, the administration will have to prove this is a real emergency, something the president seemed to undermine with his own words. I could do the wall over a longer period of time. I didn't need to do this, but I'd rather do it much faster. Those words right there, could actually come back to haunt him that he doesn't need to do this. That's been studied all day. It's not just going to be a legal challenge, but a political battle in congress. Democrats, David, are preparing to fight this in every way they can. The chairman of the house judiciary committee called it a gross abuse of power. He'll hold hearings. Democrats will issue a joint resolution to terminate the national emergency. This could well pass congress. The president could veto this. It's highly unlikely there would be the two third majority needed to override the veto. Jon Karl with us. Thank you, Jon. It's been a rough ride home. A major storm on the move.

This transcript has been automatically generated and may not be 100% accurate.