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).
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.