Sentence:  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 <WSD>cross the aisle</WSD>. 
Meaning of the word marked with <WSD> tags:  To vote, unite, or otherwise co-operate with members of another political party in order to achieve governmental or political action.