The Constitution did make it explicitly clear. The Party members have chosen to ignore it and have selected Supreme Court judges who will affirm their positions.
The Constitution provides explicit authority to the fedgov. It can only do those things specifically stated. The means for implenting the tasks are t.b.d. but the scope of the authority is not fuzzy at all.
It is a pure, unadulterated lie if anybody tries to tell you that the words "...to promote the general welfare..." assign any authority to the fedgov beyond what is explicitly stated,
Those words are in the PRE-AMBLE, which is an introduction to the list of responsibilities that follow.
It states "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."
"do ordain and establish this constitution". The rest of the Constitution defines what authority it has, not the introduction.
Politicians have been taking advantage of the illiteracy of the American people for decades now, in this regard.
They're in more than just the Preamble. They're also in Article I.8, right at the beginning of that section which outlines the powers of Congress. It's still a specious argument to say that they give Congress any extra spending powers (the phrase is only there to provide context to the power to tax), but ignoring that text altogether won't expose the faulty argument that the Left has made.