I'd suggest it is found here, "The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States." On top of this, the Constitution states, "Every Bill which shall have passed the House of Representatives and the Senate, shall, before it become a Law, be presented to the President of the United States; If he approve he shall sign it, but if not he shall return it, with his Objections to that House in which it shall have originated, who shall enter the Objections at large on their Journal, and proceed to reconsider it."
In essence, the Constitution says the House and Senate should pass the bills, then send them to the President; then if the President doesn't approve he can veto. It says nothing about not voting on bills because the President doesn't like them. While you can argue that the President, because he has the veto, should have a voice in the process, that isn't what the Constitution states. To me, a good argument could be made, based on a strict (or originalist) reading of the Constitution, that Sen. McConnell is shirking his duty under the Constitution; that the Founders always intended that the Congress be of equal power to the President, and not to be beholden or a "rubber stamp" for the President.