In politics, crossing the floor is to vote against party lines. The term originates from the British House of Commons, which is configured with the Government and Opposition facing each other on rows of benches. Votes, or divisions, are taken by entering lobbies to the left and right of the chamber to have one's vote tallied; the "Ayes Lobby" is on the Government side and the "Noes Lobby" on the Opposition side. If one wishes to vote against one's party, one must quite literally cross the floor to get to the other lobby.

The term has passed into general use in other Westminster parliamentary democracies, such as Australia, Canada, or New Zealand, even though most of these countries have semicircular or horseshoe-shaped debating chambers and mechanisms for voting without members leaving their seats. It is most often used to describe members of the government party or parties who defect and vote with the Opposition against some piece of government-sponsored legislation.

It is also sometimes used of a member who leaves their party entirely and joins the opposite side of the House, or even if he or she leaves one opposition party simply to join another. This usage exists not only in Westminister system parliaments, but also in legislatures in presidential systems.

See also

• Party switching for a similar concept in the American political system.