Midterms are typically bad news for the sitting president and his party. In all but three of the past 26 midterm elections, the party in the White House has lost seats in either the Senate, the House of Representatives or both [source: Enten]. The phenomenon even has a name — the midterm penalty.
The conventional explanation for the midterm penalty is that midterms function as a referendum on the president's popularity. If the president has performed well in the first two years of his term — or the economy has performed well, as is often the case — the damage is limited to a few seats in either chamber. But if voter sentiment has turned sharply against the president and his party, the midterm losses can be crippling.
When you look at historic job approval data and midterm results, this explanation seems to hold water. Starting in 1946, when a president has registered a job approval rating over 50 percent at the time of the midterm, his party has only lost an average of 14 seats in the House of Representatives. When his approval rating has dropped below 50 percent, his party has lost 36 seats on average [source: Jones].
Those are just the average losses. Individual midterm results make an even stronger case for the midterm penalty being linked to the president's performance. At the time of the 2010 midterms, for example, President Barack Obama's approval rating slipped to 45 percent and his party suffered heavy losses — 63 seats in the House and six in the Senate. Similarly, President George W. Bush's approval rating was a dismal 38 percent for the 2006 midterms and his party lost 30 seats in the House and six in the Senate [source: Jones].
But there are also competing theories to explain the midterm penalty. One is called the coattail effect [source: Murse]. The coattail effect is particularly strong when a president is swept into office in a landslide victory, like Barack Obama in 2008. Given Obama's popularity and wide appeal with Democrats, more people turned out at the polls in 2008 and 2012 to vote Democrat. Since most voters cast ballots along party lines, Democratic candidates down the ticket got a boost, from senators to House representatives to mayors. Those Democratic winners were, in effect, riding Obama's coattails.
During midterms, however, there is no coattail effect, or at least far less of an effect. There may be a few high-profile Senate races that draw larger numbers of partisan voters to the polls, but nothing close to presidential contests. The result is that the president's party, regardless of the president's performance or popularity, takes a hit during the midterms, because many of the voters who were excited to vote in the general election lack motivation to participate in the midterms.
A second theory for the midterm penalty is called negative voting. Political science research shows that people are more likely to vote when they have negative opinions of the president than when they have positive ones. At first, this data seems to support the theory that midterms are a referendum on the president. But there's a difference. Even if half of all voters support the president, fewer positive voters will show up at the polls compared the other 50 percent who hate him. So the midterm results may look like a resounding "protest vote" even though half the country has nothing to protest [source: Erikson].
To wrap things up, let's take a look at some of the most remarkable midterm results in U.S. election history and their lasting political consequences.