|This article does not cite any references or sources. (August 2009)|
A party-line vote in a deliberative assembly (such as a constituent assembly, parliament, or legislature) is a vote in which every member of a political party votes the same way (usually in opposition to the other political party(ies) whose members vote the opposite way). The term implies that the decision is irrelevant to the merits of the issue at hand or the political beliefs of individual members, but instead is dictated by party policy.
The United States
In the United States Congress it is the function of the party whip of each party in each house to ensure that members adhere to party policies and in particular that members vote for or against bills, amendments, and (in the case of the United States Senate) for or against treaties and Administration appointments as determined by senior party leadership. The leverage available to the party whip may be in rewards (such as the negotiation of side deals for pork barrel spending), or in punishments (such as withholding appointments to powerful committees). The ultimate threat is to support another candidate in the primary election which is a threat made possible by gerrymandering that ensures the success of a particular party's candidate. The party whip will operate under the direction of the particular party's leader (called respectively the majority leader or the minority leader).