She's a polisci major. She just got sucked into Jane's whatever because she needed the credit and going home hadn't really appealed, like, at all. Think about it, she opted to stay in the middle of the fucking desert working with math and science she she didn't really understand over going home.
And she hadn't really needed the credit. She's not going to run for office, but she is going to kick ass on Capitol Hill as an aid, so you can kiss her ass.
Hill is the one who asks her to go down to D.C.. There's something in the defense bill that Fury doesn't like (which, join the club, no one likes the defense bill, okay? It's a ball of suck wrapped in shit set on fire), and when Darcy explained that there were like, four, maybe five senator's you'd have to work over, somehow she got sent down.
And she'd sat in gorgeous offices and drank really bad coffee (though not as bad as Jane's, all hail Jane, infinite goddess of ruined coffee), and listened to old men explain why they needed to be informed any time SHIELD took a piss.
They could have sent Tony, she thinks, but Tony only comes when subpoenaed and is kind of a dick (she likes him, don't get her wrong, but sometimes she wants to kick him in the balls).
And she explains, patiently, how yes, that's totally what SHIELD wants too, but don't you think that the wording in clause 4 might be adjusted and look, doesn't this version she drafted work better?
No one reads the fucking bills, so they agree, and she gets the changes Fury wants and somehow she gets a full ride to Georgetown courtesy of SHIELD as long as she signs on to work as a congressional liaison for five years. And, well, Jane has apparently decided that rending time and space to kick her kind-of boyfriend's ass is a good idea, and Tony is a bag of dicks to Congress and gets nobody anywhere fast, so she agrees.
In three years she has a doctorate (what, she likes the sound of it: Dr. Darcy Lewis. Plus doctorates don't make the gross assholes stop looking down her tops, but it does get her foot in the door). She gets a profile in Mother Jones and a raise and a brownstone in Georgetown.
So yeah, the Avengers can go and get beat up and save the world, but she's the one that gets them the leeway to do it.