Alabama has nonpartisan registration. There is no option to affiliate with a political party on the voter registration form.
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Independent voters are allowed to vote in primary elections. All voters must choose one party's ballot to vote on, but they are not tied to that party.
A voter can decide not to participate in either party's primary and will be given the amendments/referendum ballot only.
Those who chose the Democratic ballot are held to that affiliation in the case of a runoff election.