The US Congress, the prize up for grabs in the mid terms, holds broad sway over every facet of American life and is designed as a check on the powers of the White House and US Supreme Court.
The House of Representatives has 435 voting members who serve two-year terms, while the Senate's 100 lawmakers serve six-year terms staggered such that in any given US election roughly one third are up for renewal.
Each state automatically has two senators and has a number of representatives determined by its population, and therefore up for review after each once-per-decade US Census.
By capturing the House, US President Barack Obama's Republican foes have seized broad power over US legislation and, equally importantly, key committees able to launch potentially damaging investigations of the executive branch.
That oversight power has been described as critical in a political system that tends to concentrate power in the US presidency, though in practice lawmakers' eagerness to monitor the White House has ebbed and flowed.
The US Constitution gives the Congress the exclusive power to declare war -- though presidents have often interpreted their own authority to deploy US forces as letting them send troops into combat without a formal declaration.
Lawmakers also control the US government purse, passing the annual spending bills that keep the US government open and setting tax rates -- though legislation affecting revenue must originate in the House.
The US Congress also must approve trade pacts, and the Constitution requires presidential nominees for top Cabinet posts, to sit on the US Supreme Court, or to be ambassadors to win Senate confirmation.
To become law, a bill must clear both chambers in identical form, sending it to the president to sign or veto. Lawmakers can override a veto with a two-thirds majority vote.
Lawmakers can also impeach US officials -- including the president, which it has done on two occasions in history, with Andrew Johnson in 1868 and Bill Clinton in 1999, though both were acquitted in their Senate trials.
At the other end of the spectrum, they are called upon to name post offices, and often pass symbolic resolutions of praise, notably for sports heroes, or condemnation, notably for overseas rights abusers.
The US Speaker is second in line for the presidency, behind the vice president. The new US Congress will take office in January.
