On Friday, President Trump said that he wouldn’t rule out declaring a national emergency to justify building his border wall. Speaking from the Rose Garden, he told reporters, "We could call a national emergency because of the security of our country, absolutely." He added: "We can do it. I haven't done it, I may do it. I may do it. But we can call a national emergency and build it very quickly."

In theory, that sounds like a good way to circumvent a troublesome Congress. In practice, declaring a crisis to fulfill a campaign promise is a terrible and probably illegal way to govern.

Building a wall requires money, and under Article 1, Section 8 of the Constitution, Congress controls the money. That leaves the Trump administration scrambling to find ways to grab money already appropriated by Congress for other purposes. That's exactly why on Sunday, acting White House chief of staff Mick Mulvaney told CNN's "State of the Union" that Trump had asked "every single Cabinet secretary and the Office of Management and Budget to go out and find money that can be used legally to guard the southern border."

The most likely source of those funds would be the military's budget. A declared national emergency would give the president direct authority to authorize military construction projects under federal law.

Trump would then be able to use the military, as he suggested before, to build the wall. The caveat, however, is that "such projects may be undertaken only within the total amount of funds that have been appropriated for military construction, including funds appropriated for family housing, that have not been obligated."

That means that important projects, such as updates to bases and infrastructure improvements, would see funding cuts. That would not only weaken military operations, but it would also be a blow for members of Congress with bases in their districts who had counted on federal dollars. It could become a deeply unpopular move.

Moreover, the military doesn't just have an extra $5.7 billion lying around in its construction budget. Even if the military could find a billion dollars to pull from existing projects, that money is hardly enough for the types of barriers that Trump has promised.

To push more money to the construction fund, which could then be directed under emergency powers to build a wall, is beyond the president's powers. The defense budget is approved by Congress for specific uses, meaning that money can't be diverted to other projects on a whim. It is up to Congress, not the president, to reprogram those already-earmarked dollars for construction.

The Constitution grants Congress control of the purse strings and even emergency powers wouldn't change that. A presidential declaration is not a blank check.