Twenty-second Amendment to the United States Constitution
Some argue that the 22nd Amendment and 12th Amendment bar any two-term president from later serving as vice president as well as from succeeding to the presidency from any point in the presidential line of succession. Others contend that the original intent of the 12th Amendment concerns qualification for "service" (age, residence, and citizenship), while the 22nd Amendment, concerns qualifications for "election", and thus (strictly applying the text) a former two-term president is still eligible to "serve" as vice president (neither amendment restricts the number of times an individual can be elected to the vice presidency), and then succeed to the presidency to serve out the balance of the term (though prohibited from running for election to an additional term).
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.