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.
Primary elections in the United States
In California, under Proposition 14 (Top Two Candidates Open Primary Act), a voter-approved referendum, in all races "except" for that for U.S. President and county central committee offices, all candidates running in a primary election regardless of party will appear on a single primary election ballot and voters may vote for any candidate, with the top two vote-getters overall moving on to the general election regardless of party. The effect of this is that it will be possible for two Republicans or two Democrats to compete against each other in a general election if those candidates receive the most primary-election support.