Cross The Aisle
(chiefly Britain and Canada, 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.
Example: 2006 Oct. 24, "Commentary: How this US election may help Iraq," Christian Science Monitor (retrieved 27 Sep 2008):
It's voters who seem to want Republicans and Democrats in the next Congress to cross the aisle and try something different in Iraq.
2008: Fareed Zakaria, The Post-American World, Chapter VI
The British prime minister doesn't need any support from the opposition party; he has a ruling majority by definition. The American system, by contrast, is one of shared power, overlapping functions, and checks and balances. Progress requires broad coalitions between the two parties and politicians who will cross the aisle.
1967 Sep. 26, Geoffrey Stevens, "Kind thoughts, gentle words, then House reverts to form," Globe and Mail (Canada), p. 1:
Liberal members pounded their desks in delight as Social Crediter Horace (Bud) Olson (Medicine Hat) picked up his books and papers and crossed the aisle to take the last seat in the third row on the Government side. This symbolic gesture completed his defection.
1995 Oct. 9, John Darnton, "Tories, With a Bagful of Woes, Are Hurt by a Defection," New York Times (retrieved 27 Sep 2008):
A Conservative member of Parliament crossed the aisle this weekend to join the resurgent Labor Party of Tony Blair.
2005 April 14, "Kilgour can't 'live with himself,' quits Grits," CTV.ca (retrieved 1 Apr 2009):
When asked by Duffy about rumours that he was being heavily courted by the Tories to cross the aisle and join them, Kilgour replied that they've opened "No doors."
2007 April 16, David Olive, "Belinda accomplished much in relatively short time," Toronto Star (Canada) (retrieved 1 Apr 2009):
After Stronach's 2005 defection to Paul Martin's Liberals . . . . [a]t least one Tory MP openly called her a "whore" for crossing the aisle.