Initiating a push or any other action with GitHub from the command line (over https, not ssh) that calls for the username and password not only fails but, when it does, it returns
Username for 'https://github.com': username
Password for 'https://firstname.lastname@example.org':
remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/username/repository.git/'
GitHub's support determined the root of the issue right away: Two-factor authorization.
To use GitHub over the shell with https, create an OAuth token. As the page notes, I did have to remove my username and password credentials from Keychain but with
osx-keychain in place, the token is stored as the password and things work exactly as they would over https without two-factor authorization in place.