President Donald Trump is expected to sign an executive order Wednesday to begin paying for a wall on the U.S.-Mexican border, a senior administration official told NBC News on Tuesday night, taking the first step toward fulfilling his marquee campaign promise. The official, who asked not to be identified, said Trump would likely sign the order – which would shift money from other federal programs to the wall project – during an appearance Wednesday at the Department of Homeland Security, the parent agency of U.S. Customs and Border Protection.