A Registered Republican is a voter who register's Republican in closed primary states that require voter identification by political party. Critics claim closed primary registration violates the spirit of the Australian secret ballot.
In Democratic party "safe districts" where an incumbent has no primary election challenger, it is not uncommon for Democrat activists to register as Republicans to vote against a qualified challenger and promote a weaker candidate.
Voter registration by party only applies to primary elections, and does not apply to general elections.
In many machine-dominated Democrat states and cities it is not uncommon for reform-minded Democrats to register as Republicans, as it historically has been impossible to reform the Democrat party from within, and local Republican parties may be virtually non-existent. Their Republican registration, vote, or candidacy can often be seen a protest against Democrat corruption. These voters and candidates are often characterized as RINOs.