Rails - Using twitter oAuth for logging in users

I use Twitter oAuth to allow users to signup for my site.

I have the user's access_token, access_secret from their signup stored.
When the user wants to return to the site and login to their account, how do I authenticate them using twitter login/password?

I do not want to ask users to give access like:
- give access to example.com to read/write your stuff on twitter

The user already has signed up using their twitter credentials, how do I reuse it instead of asking them to create a separate password?
I would rather not venture towards authlogic in addition to oauth

Currently, if the user is already logged into twitter, I can authenticate him.using the access_token, access_secret. What if he is not logged in? How do I prompt for username/password for twitter and authenticate for my app?

Thanks for your help.