Sentence:  The three subordinates on whom President Roosevelt depends to win his Senate victories are Majority Leader Joseph T. Robinson of Arkansas, the bull-voiced, heavy-fisted field commander; Pat Harrison of Mississippi, the shrewd committee and cloakroom <WSD>horsetrader</WSD>; and James F. Byrnes of South Carolina. 
Meaning of the word marked with <WSD> tags:  One who is skillful in transacting or negotiating of any kind.