A defense official who follows border wall issues very closely pointed out there are a number of issues the Pentagon and White House must resolve in order to make a possible national emergency declaration workable.
As CNN has reported, the Department of Defense, even without a declaration, is already working to identify the funds that are not yet under contract for military construction projects.
How they're planning to the fund the wall: Defense Department officials told CNN that the Pentagon is planning a figure of about $2.5 billion in funds they believe they can tap to support construction of a border wall if Trump declares an emergency and orders the military to build a wall. Those funds fall under the "unobligated" pool of funds, which means the funds are earmarked but have no signed contracts signed for spending that money. Anything beyond that would require the cancellation of existing military construction projects, which might come with costly termination fees.
If Trump wants additional funds from projects that are already in contract, he'd have to cancel projects like a fire station at Quantico, child development at Joint Base Andrews or Navy Seal training facilities improvements for combat training.
The official said one concern is that once the money goes to a wall: How do you get re-funded by Congress for the construction of these other projects so readiness is not impacted?
If a decision is made for a national emergency, the Pentagon will offer different courses of action to proceed.