cross the floor

verb2

1. •
   (UK) 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.

   • Two weeks ago, Mr Davies intimated to Mr Brown that he was ready to cross the floor. — 2007
2. •
   (UK) To vote against one’s own political party in parliament.