While the Constant Contact REST API currently supports several authentication models, at this point (Summer 2012) we request that developers use the OAuth 2.0 authentication model. We will be announcing the end of support for Basic Authentication soon, and we strongly encourage developers to begin switching 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.
OAuth Authentication model has been added to our authentication options. If you are building an application which will be used by multiple Constant Contact accounts, you should consider using OAuth Authentication.
The Basic Authentication model over HTTPs is one of the two methods for authentication with Constant Contact. Since this method requires you storing a password for all access, it is recommended that this is only used when you are developing an integration for your own use or the use of only a single account. This method is DEPRECATED and should no longer be used. It will be removed from the product shortly.