In the good ol' US of A
, a "midterm election" occurs every four years: the most recent midterm election was on November 5, 2002
. Midterm elections get their name from the fact that they occur in the middle of the president
's term of office.
We have midterm elections because while the President's term is four years long, the term of office in the House of Representatives is only two years long, while the term of office in the Senate is six years long. This means that every two years, the entire House and a third of the Senate are up for re-election. Since the presidency is only up for election every four years, there is always an election between presidential elections in which the legislative branch changes, but the executive branch does not.
The most famous midterm election in recent history was that of 1994, during a period of very low approval ratings for sitting president Bill Clinton. That year, Congressman Newt Gingrich (R-GA) led the Republican Party to a takeover of both houses of Congress, using the Contract with America as a nationwide platform, and pouring thousands of dollars in aid to campaign managers across the nation from the GOPAC headquarters in Washington, DC. It was the first Republican majority in the House since 1955, and the first Republican majority in the Senate since 1987: the Republicans hold their majority in the House to this day, while their majority in the Senate temporarily ended in 2001 thanks to one James Jeffords (I-VT). For Clinton, the regime change on Capitol Hill meant having to abandon some of his more liberal proposals, including national health insurance, and seeking a more centrist policy.
As Clinton's example illustrates, midterm elections are usually reflective of how the public views the administration in the White House. The 2002 elections won several more seats for the Republicans in both houses, indicating that the public is still supportive of George W. Bush's policies and doesn't seek a significant change in the status quo yet. In theory, if the sentiment against Bush was high in America as a whole, the Democrats would have been voted in to undermine the Republicans' absolute power in the government.