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How do I go about turning down a gov internship I accepted in December assuming that the SA route was closed off to me? Two weeks ago I miraculously got an interview/offer from a big law firm in my target market and now I need to turn down my government internship that I accepted a while ago. Considering that this is the difference between working in my target market, in my chosen practice group, while earning $30,000+, and living with my family cost free versus an unpaid internship where I have to pay my own COL, I have no choice but to backout. Should I say it is because of a better offer or just be vague and say "my personal circumstances have changed and I am no longer available to work in [x] city this summer."?

I don't see any reason to mention another offer and I think the "personal circumstances" response you mentioned is probably sufficient. It doesn't seem like you want to work for the agency that you're reneging on but just know that you'll likely burn that bridge going forward. Once again, probably doesn't matter because it's not you're target market, and this happens all the time so they likely won't care, but also probably won't hire you in the future.

Let them know immediately. You are extremely grateful for the opportunity but recently received your dream offer to be a summer associate in your target market and desired practice area that you cannot pass up in good conscience. Go with that or what you suggested is fine as well as they really don't care what your reason is. Trust me, this won't be their first rodeo and they'll have no trouble replacing your spot. Just be professional and courteous and it shouldn't result in burning any long term bridges. Main point is, do it soon and don't overthink it.

I think something about an offer more closely aligned with your long-term goals is good. I wouldn’t say that it’s a firm or imply that money was a consideration (of course it was, it’s just polite to pretend it’s not). I think chances are good the internship wouldn’t hire you in future but they’re certainly not going to go out and blackball you.

Just tell them. This happens to them all the time.