Not to mention it's a felony and it will be prosecuted. In your situation, you would almost surely have to know an already-registered voter's name and address

Not to mention it's a felony and it will be prosecuted. In your situation, you would almost surely have to know an already-registered voter's name and address -- I'm not aware of any states that let you register on the spot without government id. Many people don't register, and it's not automatic, so that's already an obstacle. Also, the log books your voting location maintain get marked when a registered citizen votes. So if the voter you impersonated comes and votes later, your impersonation will be discovered.