Election Day voter registration

Voter registration in the United States takes place at the county level in many states and at the municipal level in several states, and is a prerequisite to voting at federal, state and local elections. The only exception is North Dakota, which does not require registration, although North Dakota law allows cities to register voters for city elections. The majority of states set cutoff dates for voter registration, ranging from 2 to 4 weeks before an election; while a third of states have Election Day or "same-day" voter registration which enables eligible citizens to register or update their registration when they vote before or on Election Day.

It has been argued that some registration requirements deter some people (especially disadvantaged people) from registering and therefore exercising their right to vote, resulting in a lower voter turnout. According to a 2012 study, 24% of the voting-eligible population in the United States are not registered to vote, equaling some 51 million U.S. citizens. While voters traditionally had to register at government offices by a certain period of time before an election, in the mid-1990s, the federal government made efforts to facilitate registering, in an attempt to increase turnout. The National Voter Registration Act of 1993 (the "Motor Voter" law) now requires state governments to either provide uniform opt-in registration services through drivers' license registration centers, disability centers, schools, libraries, and mail-in registration, or to allow Election Day voter registration, where voters can register at polling places immediately prior to voting. In 2016, Oregon became the first state to make voter registration fully automatic (opt-out) when issuing driver licenses and ID cards, since followed by four more states. Political parties and other organizations sometimes hold voter registration drives to register new voters.

In most states, persons registering to vote may at the same time declare an affiliation with a political party.

While the United States Congress has jurisdiction over laws applying to federal elections, it has deferred the making of most aspects of election laws to the states. The United States Constitution prohibits states from restricting voting rights in ways that infringe on a person's right to equal protection under the law (14th Amendment), on the basis of race (15th Amendment), on the basis of sex (19th Amendment), on the basis of having failed to pay a poll tax or any tax (24th Amendment), or on the basis of age for persons age 18 and older (26th Amendment). The administration of elections, however, vary widely across jurisdictions.

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