Great question — answering as the UphillTech team:
NOTE: I cannot answer this without spoilers ... reader beware. There are several things going on with Doug, Rachel, and the President. Back in Season 1 of HoC, Doug Stamper pays Rachel Posner as a call girl to be with Rep. Peter Russo on several occasions. This ultimately ruined his chances of accomplishing anything in Congress and provides a means by which gets Russo out of the way and provide a hole with which to place the Vice President after Russo's death. Rachel is aware of her connection to this or at a very minimum, President Underwood believes she can figure it out. She was being paid by Doug to be the directed call girl just before Russo commits suicide (well, we know that...
Full answer by Pᴀᴜʟsᴛᴇʀ2: https://movies.stackexchange.com/a/31886 (CC BY-SA 4.0).
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