cross the aisle

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb idiomatic, politics To vote, unite, or otherwise co-operate with members of another political party in order to achieve governmental or political action.
• verb idiomatic, politics 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.