A sacrificial lamb is a metaphor

Metaphor

A metaphor is a literary figure of speech that uses an image, story or tangible thing to represent a less tangible thing or some intangible quality or idea; e.g., "Her eyes were glistening jewels." Metaphor may also be used for any rhetorical figures of speech that achieve their effects via...

ical reference to a person or animal sacrifice

Sacrifice

Sacrifice is the offering of food, objects or the lives of animals or people to God or the gods as an act of propitiation or worship.While sacrifice often implies ritual killing, the term offering can be used for bloodless sacrifices of cereal food or artifacts...

d (killed or discounted in some way) for the common good. The term is derived from the traditions of Abrahamic religion where a lamb

Domestic sheep

Sheep are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order Artiodactyla, the even-toed ungulates. Although the name "sheep" applies to many species in the genus Ovis, in everyday usage it almost always refers to Ovis aries...

 is a highly valued possession, but is offered to God

God

God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

 as a sacrifice for the forgiveness of Sin.
In politics

Politics

Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

, a sacrificial lamb candidate is a candidate chosen to contest an election

Election

An election is a formal decision-making process by which a population chooses an individual to hold public office. Elections have been the usual mechanism by which modern representative democracy operates since the 17th century. Elections may fill offices in the legislature, sometimes in the...

 despite the fact that he or she has little chance of victory.