We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.The Preamble of the U.S. Constitution can best be understood and explained as a statement of purpose for the document that follows. Its functional meaning, by the common law rules of construction, is to restrict what follows to the purposes it expresses. Thus, a delegation of power that might seem to be plenary is to be interpreted as constrained to those purposes. As such, no delegations of power in the Constitution are plenary.
For example, consider the Elections Clause. Congress is granted pre-emptive powers over state laws to prescribe the time, manner, and place of congressional elections, except the place of senatorial elections. On a plain understanding of the words, that would seem to empower Congress to require congressional elections to be conducted within a one-nanosecond timeframe, while balancing on one's fingertip, at a polling place on the moon. Obviously absurd? Of course, but if you seek a textual basis for excluding that, it can be found in the Preamble.