Transcript for Trump signals he may declare national emergency

We move on to Washington and day 21 of the government shutdown now sure to be the longest in history. Federal paychecks set for today will not be going out as president trump signals he may be ready to declare a national emergency to get his border wall. Want to go to Jon Karl and, Jon, one idea the white house is looking at diverting money from disaster relief to pay for the wall? Reporter: A very controversial one at that, George. But today is supposed to be payday for hundreds of thousands of federal workers who now will be going without pay, any pay whatsoever as we enter day 21 of the government shutdown with no obvious end in sight. President trump surrounded by border officials told Sean hannity he is so determined to build a border wall. Death is pouring through. Reporter: He is willing to go around congress and declare a national emergency to get the funds to build it. If we don't make a deal with congress most likely I will do that. I would actually say I would. I can't imagine any reason why not because I'm allowed to do it. The law is 100% on my side. Reporter: Sources tell ABC news the white house is laying groundwork to use funds allocated to the army corps of engineers potentially diverting funds meant for disaster relief projects in California, Florida, Texas, Puerto Rico and other states. Democrats are already preparing legal challenges. It's bad enough that he's preventing 800,000 people from getting their paychecks tomorrow, now he wants to take money from victims of disasters. Reporter: While visiting the border president trump seemed to reinvent one of the central promises of his campaign. Who's going to pay for the wall. 100%. Do you think they'll write a check for 20 billion or 10 billion or 5 billion or 2 cents? No, they're paying for the wall in a great trade deal. Reporter: In fact, his campaign website described a scenario where Mexico would make a one-time payment of $5 billion to $10 billion to build the wall. In a promotional video put out by the white house overnight, the president said his wall will get built even though negotiations with congress are going nowhere. The shutdown ties a record today and by tomorrow, George, will be the longest shutdown in American history. It's not just Democrats warning the president against declaring this emergency. Reporter: George, you have prominent conservatives who see this as an abuse of power who see it as setting a bad precedent that could then be used by a future democratic president, for instance, senator Marco Rubio has warned that a future democratic president could declare a national emergency to deal with climate change and there is even opposition among the president's top advisers to this idea. Thanks very much.

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