Put simply, midterm elections exist because not all elected offices in the United States federal government have four-year terms equal to the president. The terms of each elected office in the federal government were debated and settled during the 1787 Constitutional Convention and inscribed in the U.S. Constitution.
In the U.S. federal government, the only elected officials are the president, vice president and congressional representatives. Congress consists of two chambers, the Senate and the House of Representatives, each with its own term lengths and election schedules.
There are 100 U.S. senators, two for each state. Senators serve six-year terms, the longest of any elected official in the federal government. But not every Senate seat is up for election every six years. Instead, according to Article I, section 3, clause 2 of the Constitution, the Senate is divided into three "classes." In the modern Senate, each class consists or 33 or 34 senators. Senate elections are staggered so that one class is up for election every two years.
That's why every midterm election there are at least 33 or 34 senate seats up for a vote. There can be more seats up for election if a senator dies in office or resigns before the end of his or her term. The election schedule is also staggered so that no more than one senate seat from each state is up for grabs during any single election, midterm or otherwise.
According to Constitutional scholars, there were a couple of reasons why the Founding Fathers decided on a six-year term for senators with staggered elections every two years. First, senators are supposed to bring stability and continuity to the federal government. By serving six years, for instance, they may serve under two different presidents.
But that continuity comes with a price. The Founding Fathers worried that senators would become too chummy and permanently conspire for "sinister purposes" [source: U.S. Senate]. So they decided to shuffle the senatorial deck every two years and put a third of the seats up for a vote.
The House of Representatives is a different beast. Representation in the House is based on population, with more populous states getting more seats at the legislative table. The current House of Representatives has 435 members, each serving two-year terms, the shortest of federally elected officials.
The term length of representatives was another lively debate at the Constitutional Convention. The precedent, as set by colonial legislatures and early state legislatures, was that the representatives in state Houses were elected every year and sometimes every six months. The idea was that a short term kept representatives honest to the needs of their constituents. Some of the Founding Fathers believed that federal representatives should also be held to the same tight leash — a one-year term.
Others at the Constitutional Convention argued that one year was far too short to get anything done in the federal government, where legislative business would be more complicated than in the states and colonies. As a practical matter, they also pointed out that it took months for some representatives to simply travel back and forth from their home states to the capital, so they lobbied for a three-year term in the House. In the end, the two sides compromised on a two-year term [source: U.S. House of Representatives].
With two-year terms, every single seat of the House of Representatives is up for election every two years. That's why Americans vote for their House members during both presidential election years and midterms.
But midterms are not just about national races.