Once upon a time, a few decades ago, members of the U.S. Senate spent a fair amount of time together and got to know each other personally. They were more likely to stick around Washington on the weekends, and during breaks, and socialized together more often with their families.
Once upon a time, too, the party lines in the Senate were pretty fuzzy. Republican Senators from the north were often more liberal than Democratic Senators from the south. Alliances and friendship across party lines were common.