Transcript for Trump to sign border bill and declare national emergency

And we begin tonight with the president. The white house tonight saying he will declare a national emergency to get money to build his border wall. It comes as the president is expected to sign a funding bill in the coming hours that will keep the government open, but will only give president trump money for about 55 miles of fencing. He asked for 200 miles. Tonight, Democrats already calling the president's move lawless and a gross abuse of power. And they warn of what future presidents, future Democrats could do, if they do the same thing as president trump. ABC's chief white house correspondent Jonathan Karl leading us off. Reporter: After some last-minute drama, the president told congressional leaders he will indeed sign a spending bill he does not like, removing the threat of a government shutdown. He's prepared to sign the bill. He will also be issuing a national emergency declaration at the same time. And I've indicated to him that I'm going to support the national emergency declaration. Reporter: The bill provides a fraction of the money the president wants to build his border wall, but he says he will find a way to build the rest. And it's a big wall. It's a strong wall. It's a wall the people aren't going through very easy. You're going to have to be in extremely good shape to get over this one. They would be able to climb mt. Everest a lot easier, I think. Reporter: One of the president's closest allies had said he could not sign the bill without simultaneously declaring a national emergency. This is the time. That is a necessity. That would allow the administration to access billions more in wall funding. Reporter: Under a national emergency, the president will attempt to shift money from elsewhere in the federal government, likely the Pentagon, a move that will be swiftly challenged in court. Democrats call it a lawless act and a gross abuse of power. If the president can declare an emergency on something that he has created as an emergency, an illusion that he wants to convey, just think of what a president with different values can present to the American people. All right, so, let's get to Jon Karl, live at the white house tonight. And Jon, it's not just Democrats who are unhappy with the president's decision here to declare a national emergency. Many Republicans have been raising concerns for quite some time. They argue what future presidents could do, as well, for example, a democratic president after a series of mass shootings, the steps that presidents could tang. Reporter: Republicans were making this point again today, saying it sets a terrible precedent that will be cited by a future democratic president. And David, ABC news has learned that the department of justice has warned the white house that this national emergency declaration is likely to be blocked, at least temporarily, by the courts, preventing the immediate implementation, as it is challenged, although I'm told by a top white house official that the white house remains confident they will ultimately win on appeal. All right, Jon Karl who will be following this through the night, thank you.

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