cross the aisle

verb2

1. •
   (US) To vote, unite, or otherwise co-operate with members of another political party in order to achieve governmental or political action.

   • It's voters who seem to want Republicans and Democrats in the next Congress to cross the aisle and try something different in Iraq. — 2006
2. •
   (UK, Canada) 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.

   • A Conservative member of Parliament crossed the aisle this weekend to join the resurgent Labor Party of Tony Blair. — 1995