Sentence:  Liberal members pounded their desks in delight as Social Crediter Horace (Bud) Olson (Medicine Hat) picked up his books and papers and <WSD>crossed the aisle</WSD> to take the last seat in the third row on the Government side. This symbolic gesture completed his defection. 
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.