Party discipline is the ability of parliamentary members of a political party (often referred to as the caucus or parliamentary party) to get all its parliamentary members to vote in support of policies agreed to by a majority of the parliamentary members or of the party leadership.
The party usually appoints a person, commonly referred to as the party whip, whose primary task it is to maintain party discipline and to ensure that parliamentarians vote in accordance with the votes in caucus. 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.[further explanation needed][neutrality is disputed]
Breaking party discipline in parliamentary votes can result in a number of consequences such as not being promoted to a cabinet position, and losing other perks of elected office like travel. Disagreement with the party caucus may lead to a member leaving 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. There are occasions in which members of a party are granted a conscience vote or free vote, in which party discipline is waived.
"It is my will to join the Communist Party of China, uphold the Party's program, observe the provisions of the Party constitution, fulfill a Party member's duties, carry out the Party's decisions, strictly observe Party discipline, guard Party secrets, be loyal to the Party, work hard, fight for communism throughout my life, be ready at all times to sacrifice my all for the Party and the people, and never betray the Party."
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.
An independent evaluation found that MPs in the Canadian House of Commons voted the party line 99.6 percent of the time between 2015 and 2019. Canadian MPs also face intense pressure to toe the line when making public remarks inside and outside the legislature.
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.
In Italy, the Group discipline, as required by the statutes of parliamentary groups, has registered an oscillation from strong to weak over the years, defined as a pendulum, depending on the political phase.