In the DC swamp, they are isolated from the people and fed a steady stream of propaganda that drowns out and obliterates the message from the people. It is not entirely their fault that once they arrive in DC, they actually begin to believe that the people want open borders, government run health care, one-sided trade deals (against US interests), etc., because those are the messages that the Swamp bombards them with, every hour of every day. And the Swamp is as ruthless as Big Brother (of 1984) to those who will not internalize the propaganda. Once they are afflicted with Stockholm syndrome, they become unable to remember the values and interests of those who elected them.
I think it’s less about Stockholm Syndrome, and more about blackmail, bribery, and greed.