The incumbent, in politics, is the current holder of a political office. This term is usually used in reference to elections, in which races can often be defined as being between an incumbent and non-incumbent. For example, in the 2004 United States presidential election, George W. Bush was the incumbent, because he was the president in the current term while the election sought to determine the president for the following term.
In the United States, incumbents traditionally win their party's nomination to run for office. Unseating an incumbent president, senator or other figure during a primary election is very difficult. In particular, barring major scandal or controversy, about 95% of congressional incumbents win re-election to their seats. However, shifts in congressional districts due to reapportionment or other longer-term factors may make it more or less likely for an incumbent to win re-election over time. For example, a Democratic incumbent in historically conservative Texas would have less chance of winning than a Democratic incumbent in liberal New York City, because Texas has shifted away from the Democratic party in terms of voting (see also Congressional stagnation in the United States).
However, there exist scenarios in which the incumbency factor itself leads to the downfall of the incumbent. Popularly known as the anti-incumbency factor, situations of this kind occur when the incumbent has proven himself not worthy of office during his tenure and the challenger demonstrates this fact to the voters. A anti-incumbency factor can also be responsible for bringing down incumbents who have been in office for many successive terms in spite of performance indicators, simply because the voters are convinced by the challenger of a need for change.
When newcomers vie to fill an open office, voters tend to compare and contrast the candidates' qualifications, issues positions and personal characteristics in a relatively straightforward way. Elections featuring an incumbent, on the other hand, are as Guy Molyneux puts it, "fundamentally a referendum on the incumbent. Voters will first grapple with the record of the incumbent. Only if they decide to "fire" the incumbent do they begin to evaluate whether the challenger is an acceptable alternative.
Voters typically know incumbents well and have strong opinions about their performance. Challengers are less familiar and invariably fall short on straightforward comparisons of experience and (in the presidential arena) command of foreign policy. Some voters find themselves conflicted -- dissatisfied with the incumbent yet also wary of the challenger -- and may carry that uncertainty through the final days of the campaign and sometimes right into the voting booth. Among the perpetually conflicted, the attitudes about the incumbent are usually more predictive of these conflicted voters' final decision than their lingering doubts about the challenger. Thus, in the campaign's last hours, we generally tend to see "undecided" voters "break" for the challenger. See Incumbent Races: Closer Than They Appear http://www.pollingreport.com/incumbent.htm by Nick Panagakis, 1989. (There are some major exceptions, such as in the 2004 Canadian federal election where many undecided voters went to the incumbent due to an alleged fear factor of the unknown.)
In large corporations it is the incumbent who is the holder of an office, or one that occupies a particular position.