Katz, et al:
If you are having trouble with your Constanct Contact credentials authenticating the plugin, try chaning your CC username from your email address to a username that doesn't contain special characters. (e.g. <usr> firstname.lastname@example.org should be changed to <usr> jerryspizza)
You can do this by logging into Constant Contact-->My Account-->Update Account Settings (menu on right)-->User Name
I had issues for several hours today with a client site and this was the solution (even after password changes and a hsot of other things.
Hope it helps!