You are right except that laws cannot violate the Constitution unless the Supreme Court allows it. The Constitution gives states broad latitude to legislate powers not delegated to the federal government. States have every right to establish broad healthcare programs for instance although the federal government could amend the Constitution to do the same thing. In addition, states often agree on things among themselves like accepting each others professional credentials, licenses and in-state tuition rates. If 20 blue states each separately agreed to the same universal health care model and honoring each others plans, it would be another way of achieving universal healthcare without amendments or federal Republicans impeding it.
Another caveat is that the federal government has the final word on interstate and foreign matters. Eisenhower and Congress justified the interstate highway system as a defense project allowing military and other supplies to flow rapidly for in wars and national emergencies.
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