Why does Doug Stamper have so much influence over Rachel?

At this point of time Rachel is terrified of Stamper. At first she followed him out of a sense of gratitude and loyalty. After all, he did help her out when she was at her lowest point, providing her with money and a place to stay. As the whole conspiracy with Peter Russo unfolded she began to see Stamper as he really was. Ruthless and manipulative. She knew she had landed herself in very hot water indeed. She was now a pawn in a game bigger than herself, and if she wanted to stay safe she had to follow orders. She also began to suspect Stamper felt some attraction to her, which complicates matters. Rachel is smart enough to know that at least two deaths are already linked to Underwood/Stamper She had to keep him happy, or else it might be her next. All of her fears spill out of her when Stamper is taking her to a ride to an unknown destination.