cross the aisle
1. Of politicians, to unite or cooperate—especially through voting—with an opposing political party or members thereof, so as to achieve some political goal or purpose. Primarily heard in US, South Africa. With just enough Republicans crossing the aisle, the Democrats were able to pass the bill through both the Senate and the House of Representatives. If reform of any kind is going to come about in this lifetime, it will take some strong-willed individuals willing to cross the aisle.
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 aisle to the Labour Party.