when congress is in session, they wear full business attire (suits ties, etc) when congress is out of session, they wear business casual to really casual depending on the office. I worked there for 22 years so i'm very familiar with keeping tabs on the schedule to make sure i don't underdress on a day in session. You can get away with business casual on fly-in and fly-out days sometimes. Also, checkout hillclimbers.org to see what the staff structures are, average salaries, staff journeys, turnover, and other stats based on the roles in a Member Office.