All powers not granted to the federal government in our Constitution are reserved either by the people or the States. This is clearly spelled out in the ninth and tenth amendments to our Constitution.
“The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.”
“The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”
Thus, simply, the Congress, the President, or the federal courts may not take upon themselves any power not specifically delegated to it by the Constitution. While the “general Welfare” clause is often used by members of our government to do whatever they want under the guise of a large, powerful grant of power, the fact is however that if that singular clause allowed the federal government to, at any time, do anything it wanted, then the ninth and tenth amendments would be pointless for there would be no powers retained by the people. As our founders readily stated, the powers of our Constitution were few and defined, not infinite and nebulous.
The checks and balances of our federal government where each branch has authority in some capacity to check what the other does would, under normal circumstances and in sane times, be enough to prevent usurpations of the sovereignty of the people and the States. However we are certainly not in normal or sane times.
As such however the states have the ability to nullify any action taken by the federal government which exceeds the clearly defined and limited powers it is granted. This nullification is simple in that the States are under no obligation to obey an unconstitutional law. While some may pass laws and resolutions within their own borders to make it official, even that step is not necessary.
Even if some states stupidly accept an unconstitutional law’s power and abide by it, no other state is under any obligation to do the same.
It should be noted that State’s rights and their ability to nullify laws do not, in any way, extend an instance where they are found to be violating the inalienable rights of their citizens. The Civil War is a prime example of this. The Confederacy claimed that their secession was legal and just because of States Rights due to the northern states and their hostility to slavery. However their concept of States Rights was that they, as States, had the right to violate the inalienable rights of those in their care through the institution of slavery. This is a vacuous argument and there is not now, nor ever was or will be a right for a State to violate the inalienable rights of those within its borders based on a concept of States Rights.