Twenty-second Amendment to the United States Constitution
The amendment prohibits any individual who has been elected president twice from being elected again. Under the amendment, an individual who fills an unexpired presidential term lasting greater than two years is also prohibited from winning election as president more than once. Scholars debate whether the amendment prohibits affected individuals from succeeding to the presidency under any circumstances or whether it only applies to presidential elections.
United States presidential primary
In many states, only voters registered with a party may vote in that party's primary, known as a closed primary. In some states, a semi-closed primary is practiced, in which voters unaffiliated with a party (independents) may choose a party primary in which to vote. In an open primary, any voter may vote in any party's primary. In all of these systems, a voter may participate in only one primary; that is, a voter who casts a vote for a candidate standing for the Republican nomination for president cannot cast a vote for a candidate standing for the Democratic nomination, or vice versa. A few states once staged a blanket primary, in which voters could vote for one candidate in multiple primaries, but the practice was struck down by the U.S. Supreme Court in the 2000 case of "California Democratic Party v. Jones" as violating the freedom of assembly guaranteed by the First Amendment.