Party-line vote

A party-line vote in a deliberative assembly is a vote in which a substantial majority of members of a political party vote the same way.
Sources vary on what proportion of party members must adhere to the party line in order for the vote to constitute a "party-line" vote. For example, in the United States, the Congressional Record has stated: "A party-line vote is one on which a majority of Republicans vote one way and a majority of Democrats vote another. 2. A bipartisan vote is one in which a majority of Republicans and a majority of Democrats vote the same way". Another source defined this event for purposes of classifying votes for research purposes as "one where 90 percent of the majority party votes against 90 percent of the minority party". Party-line votes are also noted to reflect the degree to which the division of power requires parties to retain cohesion in order to implement its goals:

United States

In the U.S. 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 for or against treaties and administration appointments as determined by senior party leadership. The leverage available to the party whip may be in rewards or in punishments. The ultimate threat is to support another candidate in the primary election with endorsements and party funds. The party whip will operate under the direction of the particular party's leader.
Prior to the 1980s, it was not uncommon for members of the U.S. House of Representatives to "cross the aisle" and vote for legislation supported by the opposing party. By the mid-1990s, however, the House became almost completely partisan and polarized.