To the ASP.NET Membership API the "user's login" is the UserID.
If you want to programmatically validate a user try:
Membership.ValidateUser(username, password)
And then to retrieve the user object try:
Membership.GetUser() <--- gets current user
Membership.GetUser(username) <--- if you want a user other than the current.
There are only 10 types of people in the world: Those who understand binary, and those who don't