Is there an API by which i can associate a contact list to a campaign. This can be done using the GUI, but is it possible via an API?
This can be done through our API. Both for emails created through our API and for emails created through the website. Here is where you can find the API documents detailing how this can be done.
Pay specific attention to the data pair for "sent_to_contact_lists" as that is how you specify which list your email will be sent to.
For emails created through the Constant Contact website. http://developer.constantcontact.com/docs/email-ca
For emails created through the API. http://developer.constantcontact.com/docs/email-ca