From the website - visit http://www.casey.senate.gov/request/internships to learn more:
"Senator Casey is pleased to offer internships in his Washington, D.C. and Pennsylvania offices to undergraduate and graduate students during the spring, summer and fall semesters. Interns will gain first-hand knowledge of the legislative process and of the daily operations of a Senate office by working with constituents and staff on issues affecting Pennsylvania. While our office welcomes all applicants, preference will be given to candidates with a Pennsylvania background.
Press and legislative internships, along with the Sullivan and Higginbotham internship, are only available in the DC office. Constituent services internships are available in the State offices."