(CNN) With a simple statement on the Senate floor, Mitch McConnell set a hugely important precedent on the relationship between the legislative and executive branches.

In announcing that President Donald Trump would sign the compromise legislation to keep the government open past Friday , McConnell also expressed support for Trump to declare a national emergency on the border -- a move that will allow the chief executive to tap into funds already allocated by Congress for other purposes to bridge the gap between the funding for his border wall in this compromise deal ($1.375 billion) and the amount Trump says he needs ($5.7 billion).

Why does that matter so much? Because what McConnell is doing is effectively ceding Congress' power to allocate money. He is saying that on this one issue -- in order to get what he wants (the government staying open) he is willing to allow Trump to declare a national emergency and, thereby, take money Congress has dedicated to some other purpose to use for something the President considers more of a priority.

"Oh, who cares?" you say. "There are billions of dollars swimming around all over the federal government. What difference does it make if McConnell lets Trump use a few billion that was supposed to be spent elsewhere?"

The Constitution cares. It's right there in Article 1, Section 9, Clause 7 : "No Money shall be drawn from the Treasury but in Consequence of Appropriations made by Law; and a regular Statement and Account of the Receipts and Expenditures of all public Money shall be published from time to time."

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