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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.
The term has a somewhat different meaning in Marxism–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.
Breaking party discipline in parliamentary votes can result in a number of penalties for the member who dissents, These penalties include not being promoted to a cabinet position, and losing other perks of elected office like travel. Their disagreement with their party caucus may be so strong that they leave the party to join another parliamentary caucus or become an independent. This is known as crossing the floor.
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
Party discipline tends to be extremely strong in Westminster systems such as the United Kingdom, Canada, Australia, New Zealand and India in which a vote by the legislature against the government is understood to cause the government to "collapse," according to the convention of confidence votes. In these situations, it is extremely rare for a member 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. Other examples of even stronger party discipline include the French Section of the Workers' International (SFIO) and the Communist Party.
In countries such as New Zealand, which use an MMP (mixed-member proportionality) system of voting, party discipline tends to be high. This is especially true for list MPs, who do not represent an electorate. If they do not vote the party line, they risk being demoted on the list in the next election and not returning to parliament.
Weak party discipline
Weak party discipline is usually more frequent in parties of notables than in populist parties. The French Radical-Socialist Party had no party discipline, neither did any of the right-wing parties during the Third Republic (1871-1940). In the US, the modern day Democratic Party and Republican Party also have only weak party discipline, though this varies somewhat between states. This is aptly illustrated by the vote on the Justice Against Sponsors of Terrorism Act, in which the only Senator to vote against overriding President Obama's veto was Chuck Schumer, the nominal Democratic leader.