Transcript for Trump's emergency declaration to spark legal challenges

Cecilia and George. Jon, thank on. We want to bring in Mary Bruce. Let me begin with you on some of the political challenges. We've seen house speaker Nancy Pelosi say she's going to challenge this. Reporter: Look, George, congress does have the power to undo this and this morning Democrats are preparing to try and do just that. They can introduce a resolution to terminate the president's national emergency and given the outcry that we're seeing from Democrats it does seem likely it would pass in the house but that would then put Republicans in the senate in a very tricky position forcing them to go on the record on this and, George, while Republican leadership is now on board, there are members of the president's own party that I've talked to who say this sets a terrible precedent, that it is a mistake even saying it is a violation of the constitution. But, Mary, even if some Republicans defect they won't have enough votes to override -- Reporter: If they want to thwart the president from making this move they're going to need enough votes to override that veto and that is a very, very high bar. Jon, let's look back at the legal challenges. You were talking about the white house view. If congress passes this resolution, the house could sue and that's just one of many legal challenges that are possible. Reporter: Oh, absolutely, George. The attorney general of California, for instance, has already said he will go to court to fight it. You will have certainly challenging coming from those on the border, land owners affected by it, even environmental challenges. This will be tied up in the courts from T minute the president signs it. That means they could block the construction of the wall for some time as it works its way through the courts, almost certain to go to the supreme court. Reporter: Yes, what the justice department said in their warning is that you are likely to see at least a temporary injunction stopping this as the court challenges go forward but, remember, George, the president has faced challenges like this before in his use of executive action including with the travel ban and in that case he ultimately won at the supreme court. Lots of fights ahead. Thanks very much.

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