Sentence:  A voting bloc in Congress can represent a long-standing alliance or can be created anew as a result of the increased saliency of a particular issue, such as war or a severe economic downturn. A division between voting blocs is called a <WSD>cleavage</WSD> and because voting blocs are impermanent, dominant <WSD>cleavages</WSD> are always changing. 
Meaning of the word marked with <WSD> tags:  The division of voters into voting blocs.