Electability is the perceived potential of a political candidate to appeal to enough of the electorate to get elected.
Electability concerns often lead a party's core supporters into doubt over which candidate to back - the one that appeals to them the most, or the one that may be more electable. As a result, in extreme cases, they may end up backing a candidate whom they only dubiously support.
In the United Kingdom, electability was a prime concern among the (opposition) Labour Party during the 1980s and early to mid-1990s. Party leaders including Michael Foot and to a lesser extent Neil Kinnock were very popular with the party's core support of trade unions and old-fashioned socialists and social democrats, but struggled to attract votes from outside these groups. They were often ridiculed by the media and perceived as unrealistic, questionably competent, and driven by ideology rather than tangible pragmatism. After five successive defeats in general elections, Labour turned to the more moderate and "electable" Tony Blair, against the wishes of many die-hard Labour Party members, and were duly elected in a landslide victory in 1997. It then became the turn of successive Conservative Party leaders William Hague, Iain Duncan Smith and Michael Howard to be perceived as "unelectable" to greater or lesser degrees.