In this case, the issues that you are running into is because you are attempting to use Basic authentication with the Constant Contact V2 API. With our V2 API, we specifically only support using OAuth as a method for authenticating your requests. You can implement this by getting an access token and then adding this header to your HTTP request:
Authorization: Bearer ACCESS_TOKEN
In order to get an access token, you can follow the directions for a single-user integration here: http://developer.constantcontact.com/docs/developer-guides/authentication.html
If you have any follow-up questions, please feel free to reply in this thread!
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