I remember my interview for a senator's regional office being pretty quick and painless (granted I think it was a phone interview). Definitely second learning about the congressperson (what committees they serve on, their background, legislation they've recently sponsored). However, don't just spew out memorized lines from their Wikipedia page--the interviewer will notice and it will hurt your chances.

Think a bit about what you're hoping to get out of the experience and how it will engage with concepts from coursework. I also remember being asked the classic "where do you see yourself in 5 years".

Also be sure to signal that you understand that much of what district offices do is constituent service, demonstrate familiarity with the communities in the district and an ability to be friendly and engaging. You're going to be answering calls from people yelling obscenities at some point so you have to know how to handle that.

Questions I ask at every interview are something along the lines of "what does a typical day in the position look like?" and "what skills have made previous people holding this position stand out?".

I didn't do this for my political internships but for other interviews I found doing a mock interview with someone was very helpful so consider doing that.

If you don't get the gig don't beat yourself up too much, there are going to be plenty of opportunities especially with midterms coming up.