You could locate a zip code/congressional district file to map the zip codes to congressional disricts to get your subtotals. The census site has files like this per state - Congressional Districts Relationship Files (State-based) - Geography - U.S. Census Bureau
You can then create a Join to your data by zip code and group by Congressional district. Remember to assign the geographic role of Congressional Distritct to the field in the data. Note that in the Census data, the states are numerical so if that's the route you go, you will have a data clean up /normalization project to do. I just provide one way to do this.
I did a quick example of Superstore Sales by Congressional district for the State of Washington in the attached viz.
i hope that helps.
This viz is in Tableau 10.1
Thanks for coming back and marking the post correct Camila.