Sentence:  After Stronach's 2005 defection to Paul Martin's Liberals . . . . [a]t least one Tory MP openly called her a "whore" for <WSD>crossing the aisle</WSD>. 
Meaning of the word marked with <WSD> tags:  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.