The Supremacy clause only applies to actions that the federal government takes legitimately, within the powers that the Constitution delegates to it.
Sure. But it's the job of the judiciary to determine whether a law or action is constitutional. The states have no mechanism to arbitrarily declare that "X is unconstitutional."
History also clearly shows us that in many cases, the states object to something for political reason rather than out of an enlightened concept of the separation of powers. E.g. the state of Alabama ultimately was not justified in upholding and promoting segregation, and was happy to use "state's rights" as a political prop to continue a variety of harmful racist policies. To imagine that state governments are not every bit as political and angling to increase their power and influence as the federal government, is naïve.
Our federal government now does many, many, many things which fall well outside its legitimate power.
That's
your opinion. And while you are entitled to your opinion, the reality is that there is no objective or impartial way to determine whether a law or action is "constitutional." The Constitution and the laws were written by human beings; the framers were pretty much all politicians, with their own conflicting agendas; they were in many respects written with the intention of granting future generations latitude; and it us up to us, the living, to decide how we want our government to operate.
And like it or not, sometimes we just have to disobey the Constitution. I realize, of course, that few people actually like to admit it, but sooner or later everyone does it or demands it. The Constitution is not perfect; it was written not just by human beings, but politicians who had no choice but to strike ugly compromises to get the thing done. The framers are no longer alive, could not have predicted the precise challenges facing future generations, and... they're dead. They don't have to live with the consequences of policy decisions allegedly made in their name.
And yet you're eager and willing to grant the federal government the power to pass laws which blatantly violate the Constitution, and to declare by fiat that these laws are Constitutional.
Incorrect.
Neither the legislative nor executive branches can make a binding declaration that their actions are constitutional; I'm explicitly stating that federal and state laws are subject to judicial review.
In turn, the courts are subject to several checks and balances, including appointment by the Executive Branch, confirmation by the Legislature, the possibility of impeachment, and the option of amending the Constitution. It's not perfect, but it's a decent option.