cross the floor

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crossing the floor

cross the floor (third-person singular simple present crosses the floor, present participle crossing the floor, simple past and past participle crossed the floor)

(UK politics, of a member of a parliament) To vote against one’s own political party in parliament.
(UK politics, informal, of a member of a parliament) To resign from one’s political party and join another party, resulting in moving from one’s currently assigned desk or seat in the legislative chamber to a new desk or seat physically located with the other members of one’s new party.
- 2007 June 27, Philip Webster, Francis Elliott, “How Brown led his latest recruit across the floor”, in Times of London, retrieved 9 Nov. 2008:
  Two weeks ago, Mr Davies intimated to Mr Brown that he was ready to cross the floor.

Potentially applicable to any parliament structured according to the Westminster system (and only such parliaments).