Separation of powers in the US Constitution is a method for providing checks and balances between the different branches of the Federal Government:
- The Legislative Branch, where the House of Representatives and the Senate have distinct powers and are apportioned quite differently to balance the power of large and small states
- The Administrative Branch, including powers of the President and Vice President
- The Judicial Branch, including the Supreme Court.
In this design it is intended that each branch shall act as a check on the powers of the other branches and a balance to them. Thus only Congress has the power to pass bills into law; only the President can veto a bill; and only the Supreme Court has the power (not explicit in the Constitution, but generally accepted) to declare laws and administrative actions Unconstitutional, and therefore null and void.
The Constitution also provides for checks and balances between State and Federal power. Later amendments to the Constitution modified the checks and balances against abuses of power by the States, applying Federal civil rights standards to them.
The ultimate protection against usurpation in any branch of government is public opinion. The people have the right in any election to "throw the rascals out", or to call for impeachment of the Vice President, the President, and various Cabinet members, as we are doing. The people also have the ability to vote in tyrants and approve of their tyrannies, right up to the point at which they give away their voting power and have no further say in the matter.