Party discipline is the ability of a parliamentary group of a political party to get its members to support the policies of their party leadership. In liberal democracies, it usually refers to the control that party leaders have over their caucus members in the legislature. Party discipline is important for all systems of government that allow parties to hold political power because it determines the degree to which the governmental infrastructure will be affected by legitimate political processes.
Breaking party discipline in parliamentary votes can result in a number of penalties such as not being promoted to a cabinet position, and losing other perks of elected office like travel. Disagreement with the party caucus may be so strong that they leave the party to join another parliamentary caucus or become an independent, which is known as crossing the floor. With party discipline, there is an unwritten rule that pressures parliamentarians to compromise their beliefs if they conflict with the decisions made by the rest of the party.
In many political systems, a member of each party is officially designated or elected as a "whip", whose role it is to enforce party discipline.
Strong party discipline
The term has a somewhat different meaning in Marxist–Leninist political systems such as the People's Republic of China. In this case it refers to administrative sanctions such as fines or expulsion that the Communist Party can impose on its members for actions such as corruption or disagreeing with the party.
Party discipline tends to be strong in countries using the Westminster system, such as the United Kingdom, Canada, Australia, New Zealand and India, in which a vote by the legislature against the government is understood, by convention, to cause the government to "collapse". Thus, it is rare for members to vote against the wishes of their party. Party leaders in such governments often have the authority to expel members of the party who violate the party line.
In countries such as New Zealand, which use the mixed member proportionality system of voting, party discipline tends to be high. That is especially true for list MPs, who do not represent an electorate. If they do not vote the party line, they risk losing their seat.
Weak party discipline
Weak party discipline is usually more frequent in parties of notables and elite parties than in populist parties. The centrist Radical-Socialist Party and the right-wing parties during the French Third Republic (1871–1940) all had no party discipline.
In the United States, the modern Democratic Party and Republican Party both have weak party discipline, but that varies somewhat between states. That is aptly illustrated by the vote on the federal Justice Against Sponsors of Terrorism Act, in which the only senator to vote against overriding President Barack Obama's veto was the retiring Democratic minority leader Harry Reid.