[2] 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: Whether a party-line vote appears on an issue reflects incentives presented by majority rule.
If, in contrast, one party has a substantial majority, some position-taking defections can be permitted.
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 with endorsements and party funds.
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.