D will be interning in DC for the summer (congressperson’s office). We’ve visited in the summer and know it is hot and humid. She is a rising senior student/athlete whose wardrobe consists mainly of golf attire, t shirts and jeans. She has one good black suit, 3-4 cotton collared shirts and wants to shop for more items. What else should she add to her collection?
I’ve had several tours led by summer interns and they have all been in suits or skirts with jackets. And pantihose. I noticed those because my girls wear them to work and seem to be in a minority these days.
Pantyhose is a must for my D. She has to hide the golfer’s tan on her feet. 
The buildings are air-conditioned so she’ll want a jacket. She can probably get away with scooped neck blouses with shorter sleeves. Some nice plain dresses can also be dressed up with a blazer or jacket. My son leaves a suit jacket at work and then walks to the train station in just a shirt. That might be an option for her.
I do suggest checking with the office if they have a formalized dress code before going shopping.
My D worked in a Senator’s office as a post-graduate intern. While giving Capitol tours she always wore a jacket…sometimes with a skirt, sometimes with a dress. The jacket was the non-negotiable piece of the wardrobe. For her, the issue was always what shoes to wear. She’s most comfortable in flats, but often felt the need to wear some sort of heel. I think she finally compromised with dressier flats. As to pantyhose, pretty sure she didn’t wear them for that job. The only time she’s felt compelled to wear pantyhose was during interviews for summer associate jobs in law firms. And from what she’s been told, that will pretty much be the last time she needs to wear them - hose are not “required” at her firm.
I highly recommend getting a second suit. If her only suit gets ripped or if she spills something on it, she might not be able to repair it or get it cleaned quickly. The Limited and Express have inexpensive coordinating pieces; I recommend getting 3 bottoms and 2 jackets in the same color or a couple of skirts and a jacket that can be worn with her existing pieces.
For flats, get some with a tiny wedge heel built in. The feet will appreciate this a lot. 
My niece did this type of internship. Any pictures I saw of her, she had different scarves on. I think she had a limited wardrobe and scarves changed it up a bit.
My daughter did this too, for a Congressman. Her office was very casual. I’m pretty sure she wore jeans some days – she definitely did not wear pantyhose (I don’t think she owns any). She didn’t give tours, though. I think checking with the office would be a good idea.
I agree that pantyhose are not required. I worked in Baltimore for a few years and it is HOT in the summer. I wore them (1980) but the deputy general counsel did not. I perfected the art of taking them of while driving home (no AC in my car).
I recommend black pants, 6 shirts, and a blazer. Flats are fine.
Jones New York outlets have great inexpensive suits, with skirts or pants for college-aged interns. A pale grey suit, and a light blue seersucker suit would be a good addition to the black suit. They carry sleeveless collared blouses nice for under the jackets. My DD never wore pantyhose for her senate office internship. Wore flats or low wedges for the miles of walking.
Daughter spent a semester interning at a Congress woman’s office in DC. Jacket was daily requirement. She found a couple of lightweight ones and wore them over breathable, but business dresses or with skirts. It is hot, subways are not always friendly to very light colors and it took a little planning. She wore the same three jackets in different ways. I believe the dress code of her office was clearly articulated prior to her arrival.
You should have her read Caphillstyle blog. It was written by a woman who worked on the Hill (I believe she worked for a Republican congressman from Wyoming IIRC) and lots of her advice was tailored specifically to DC.
Suits, skirts with jackets or sweaters, dresses. IOW professional.
My D2, working for an asset mgmt. firm in Boston, has to dress the same.
No cleavage.
Ditto on the blog mentioned by pizzagirl…it was an invaluable source of practical info and advice for my D when she was working in DC. She felt the need to be “business-ready” because she was often sent to cover hearings on the Hill with little notice. Same goes for tours - she didn’t always have advance notice. Her preference was for dresses with a jacket - that way she didn’t have to worry about tops and in the heat of a DC summer, she hated tucking things into a waistband!
NY and Co, Limited, and Dress Barn have inexpensive stuff. YMMV
Thanks everyone!