Election day registration (also called same-day registration) is the practice of allowing voters to register on the day of an election and then immediately cast a ballot. In other states, eligible voters must register a specified number of days before an election to vote in that election.
If a voter arrives at a polling place and is not already on the voter rolls, the election officials verify the voter's identity and add the name to the voter database. The newly registered voter can then cast a ballot in that day's election. Because the polling places are all connected to a central voter database, the system prevents voters from registering at more than one location or voting multiple times in the same election. Prior to the advent of reliable and secure computer networking, the only way to prevent the possibility of multiple registrations and votes was to freeze the voter list days before the election and then physically transport the list of registered voters to individual polling places so that voters could be manually checked.
Eight states currently have some form of Election day voter registration: Idaho, Iowa, Maine, Minnesota, Montana, New Hampshire, Wisconsin, Wyoming, and Washington DC. Montana began Election day voter registration in 2006, and Iowa in 2008. Connecticut and Rhode Island also have Election day registration for only presidential elections. Connecticut expanded to municipal elections in 2013. California will start in 2015 once it has implemented a statewide voter registration database. (North Dakota, unique among the states, has no voter registration requirement at all.)