The three branches of the Federal Government of the USA - the legislature, the executive and the judiciary are totally separate. No one is allowed to be a member of more than one branch. Thus when Obama offered Clinton the post of Secretary of State she had to give up her seat in the Senate. The same applied when in 1989 Bush made Cheney, a member of the House of Representatives, his Secretary of Defence. When the Democratic ticket was elected Biden had to give up his Senate seat to become Vice President. As VP he notionally presides over the Senate but he is not a member of it. In practice he does not even do the presiding, the President pro tempore does. The VP only votes if the Senate is tied as with Cheney in the first few months of 2001, until Jeffers defected. Each branch continually checks and balances the others. The President may suggest Bills as with Bush and the privatisation of social security in 2005 only to find Congress rejecting it. The President may veto a bill only to have Congress overturn it as happened to Bush in November 2007. The Congress declare war but the President wages war. The President suggests a budget but the Congress may refuse to pass it as with Bush in 1990 and Clinton in 1995. The Congress determines the number of Judges on the Supreme Court and it decides which Government Departments to create as with Homeland Security after 9/11. The President nominates to the Federal Judiciary and to the Executive branch but the Senate may reject them as they did with Bork in 1997 for the Supreme Court and John Tower in 1989 for the Bush Cabinet and Zoe Baird in 1993 for the Clinton Cabinet. The aim of all of this is to avoid a concentration of power whether it be an Imperial Presidency, an over powerful Congress or an Imperial Judiciary and it could be argued that this aim has been achieved.
In 1787 the Founding Fathers compromised between a unitary state and a Confederate one and created the Federal system of government which survives to day. Then there were 13 states, today there are 50 with their own political systems, their own laws, their own constitution and their own judges including State Supreme Courts. The rights of the states are guaranteed by the 10th Amendment, the reserved powers. Each state provides its own form of welfare ranging from generous in Wisconsin to miserly in Alabama. Some states, mostly in the South, use the death penalty, most states do not. Some states make it harder for a woman to have an abortion than others. Some have a state income tax, others eg Florida, do not. Wherever they live all US citizens enjoy entrenched rights which the Supreme Court upholds through judicial review. As William Story points out Brown v Board in 1954 saw the Warren Court rule segregation unconstitutional; Roe v Wade in 1973 saw the Burger Court prevent states from banning abortion and Lawrence v Texas in 2003 saw the Rehnquist Court uphold the rights of homosexuals. The Pentagon Papers case 1971 upheld freedom of the press; Engel v Vitale 1963 banned school prayer; Miranda v Arizona 1966 upheld the rights of suspects; Bakke v University of California in 1978 upheld the rights of a ‘victim’ of affirmative action.