The Constant Contact REST API currently supports the OAuth 2.0 authentication model. Basic Authentication is no longer supported, and we strongly encourage developers to switch to OAuth 2.0 as soon as possible. Both the Server and Client flows from the OAuth 2.0 spec are now supported, so developers can choose the best approach for their application.
Constant Contact now supports both the OAuth 2.0 client and server flows. These flows enables web and other application programmers to take advantage of this simpler form of OAuth authentication. Click the link above for details on how to get started