As it was founded by the unions to represent the interests of working-class people, Labour's link with the unions has always been a defining characteristic of the party. In recent years this link has come under increasing strain, with the RMT being expelled from the party in 2004 for allowing its branches in Scotland to affiliate to the left-wing Scottish Socialist Party. Other unions have also faced calls from members to reduce financial support for the Party and seek more effective political representation for their views on privatisation, public spending cuts and the anti-trade union laws. Unison and GMB have both threatened to withdraw funding from constituency MPs and Dave Prentis of UNISON has warned that the union will write "no more blank cheques" and is dissatisfied with "feeding the hand that bites us". Union funding was redesigned in 2013 after the Falkirk candidate-selection controversy.

What is a defining characteristic of the Conservatives?