The power of the government is limited by the Constitution; the government is given only the powers specifically written in the documents.
The three branches of government (legislative, executive, judicial), each has its own powers, duties, and limits.
Each branch checks the powers of the other two branches. The Legislative Branch (Congress) has oversight over the other two branches.
Power is divided between the federal government and the state governments.
Under the U.S. Constitution, some powers belong exclusively to the federal government, meaning state government has no constitutional power to engage in these acts:
"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."
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