You can accomplish this by running a Query to target constituents who are categorized under the legislator's district and who also has an Action Alert Response Interaction recorded in their Constituent360 profile.
  1. Go to Data Management > select Query
  2. Click Create a new Query
  3. Give your Query a name and description
  4. Click the Drop-down menu for Field Type and select Address
  5. Click the Drop-down menu for Field and select Congressional District (Automatic)
  6. Click Add Field Clause and enter the appropriate district number for the legislator
  7. Click Add An Interaction Clause
  8. Click Referenced Types > select Action Alert Response
  9. Choose your Action Alert from the list under Referenced Names
  10. Save the query and run it.
You can now use these results to create a new group or export the list using the mail merge function.