The criterion has always been "proof of citizenship" it just hasn't been rigorously enforced. I can remember crossing into Montana in the 1980's, showing my driver's license and being lectured by the border officer that "This means you are allowed to drive, it doesn't tell me your citizenship".
She let me in anyway.
In the 90's crossing into Montana again, I offered my passport but they asked for my driver's license and went in and checked it on the computer - I guess they didn't have anything else to do.
Its getting tougher and tougher as opposed to Europe where the borders seem to be disappearing.
Take your passport. Always.