cross the floor
1. Of parliamentary members, to vote against the common trend of one's political party (i.e., vote "against party lines"). Primarily heard in UK, Australia, New Zealand. Though it is largely opposed by the Conservative Party, enough PMs have crossed the floor on this issue that it looks like it might succeed.
2. Of parliamentary members, to leave one's current political party and join another, such that one's legislative seat is physically moved to the new party's location in parliament. Primarily heard in UK, Australia, Canada, New Zealand. Following the prime minister's controversial tax bill, several members of his party crossed the floor to the Labor Party.